Question 1

What is Docker and how does it implement containerization?

Accepted Answer

Docker is an open-source platform that automates the deployment of applications inside lightweight, portable containers. Unlike Virtual Machines that virtualize the entire hardware layer and run separate guest OSs, Docker containers share the host operating system's kernel, isolating applications at the process level using Linux namespaces and cgroups, resulting in faster startups and low resource overhead.

Question 2

Explain Docker Images, Containers, and Registries.

Accepted Answer

- Image: A read-only template containing the application code, runtime libraries, and environment configurations needed to run a container.
- Container: A runnable instance of a Docker image, executing in an isolated process namespace.
- Registry: A storage service (like Docker Hub) used to upload, distribute, and download images.

Question 3

What is a Dockerfile and what are its common instructions?

Accepted Answer

A Dockerfile is a text document containing instructions to build a Docker image. Common instructions include:
- `FROM`: Sets the base parent image.
- `COPY`/`ADD`: Copies local files into the container.
- `RUN`: Executes commands (like installing packages) during builds.
- `ENV`: Sets environment variables.
- `CMD`: Sets the default command executed when the container starts.

Question 4

What is the difference between CMD and ENTRYPOINT in a Dockerfile?

Accepted Answer

- `ENTRYPOINT` defines the executable command run when the container starts, which is not easily overridden.
- `CMD` provides default arguments for the `ENTRYPOINT` or acts as a default command. If you run `docker run image arg1`, it overrides `CMD` but appends `arg1` to `ENTRYPOINT`.

Question 5

Explain Docker Volumes and why they are necessary.

Accepted Answer

By default, files written inside a container are stored in a writable layer and are lost when the container is deleted. Docker Volumes are directories hosted on the host machine that bypass the container filesystem, enabling data persistence (like database records) and sharing data between containers.

Question 6

What is the difference between Bind Mounts and Volumes?

Accepted Answer

- Bind Mounts: Map any directory on the host machine to a container path, which is dependent on the host's directory structure.
- Volumes: Managed entirely by Docker in a dedicated storage directory, which is safer and independent of host filesystems.

Question 7

What is Docker Compose and when is it used?

Accepted Answer

Docker Compose is a tool for defining and running multi-container Docker applications. Using a YAML file (`docker-compose.yml`), you configure application services, networks, and volumes, launching all containers with a single command: `docker-compose up`.

Question 8

Explain Docker networking: bridge, host, and none drivers.

Accepted Answer

- `bridge` (Default): Creates a private internal network, isolating containers while allowing them to communicate via IP or container names.
- `host`: Bypasses network isolation, sharing the host machine's network stack directly.
- `none`: Disables all container networking, isolating it completely.

Question 9

How do you check container logs in Docker?

Accepted Answer

Use the `docker logs container_id` command. Add the `-f` flag (`docker logs -f id`) to stream logs in real-time as they are written by the application.

Question 10

Explain how to build and run a container in a single command line.

Accepted Answer

First, build the image: `docker build -t app:latest .`. Then, run the container: `docker run -d -p 8080:8080 --name myapp app:latest`. The `-d` flag runs the container in detached background mode.

Question 11

What is the purpose of the .dockerignore file?

Accepted Answer

The `.dockerignore` file lists files and directories (like `node_modules` or `.git`) that should be excluded from the build context sent to the Docker daemon, reducing build times and image sizes.

Question 12

Explain the difference between COPY and ADD instructions.

Accepted Answer

Both copy files. `COPY` only supports copying local files into the container. `ADD` includes extensions: it supports fetching files from remote URLs and automatically extracts compressed files (like tar/zip) during copies.

Question 13

What is the docker ps command and how is it used?

Accepted Answer

`docker ps` lists running containers, showing their IDs, image names, commands, creation times, status, and port mapping configs. Add the `-a` flag to list all containers, including stopped ones.

Question 14

How do you stop and delete stopped containers?

Accepted Answer

Stop a container: `docker stop id`. Delete it: `docker rm id`. To clean up all stopped containers, unused networks, and dangling build caches in one go, run `docker system prune`.

Question 15

What is the difference between an image layer and a container writable layer?

Accepted Answer

Image layers are read-only templates. When a container runs, Docker adds a thin writable layer on top. All modifications (creating, updating, deleting files) are written to this container layer, leaving the base image unmodified.

Question 16

Explain port mapping (port publishing) in Docker.

Accepted Answer

Port mapping maps a host machine port to a container port: `docker run -p 8080:80`. This routes incoming traffic on host port 8080 to container port 80, making container services accessible externally.

Question 17

What is the role of the EXPOSE instruction in a Dockerfile?

Accepted Answer

The `EXPOSE` instruction is documentation. It informs developers which ports the container services listen on, but does not actually publish or map ports; port mapping must still be configured at runtime.

Question 18

Explain Multi-stage Builds in Docker and how they reduce production image sizes.

Accepted Answer

Multi-stage builds use multiple `FROM` instructions in a single Dockerfile. You use a heavy build environment to compile code, and copy only the compiled binaries or static assets into a lightweight runtime image: 
```dockerfile
FROM node:20 AS builder
WORKDIR /app
COPY . .
RUN npm run build

FROM node:20-alpine
WORKDIR /app
COPY --from=builder /app/dist ./dist
CMD ["node", "dist/main.js"]
```
This keeps compilers and source files out of the production image, reducing sizes by up to 90%.

Question 19

Explain Docker container resource limits: configuring CPU and memory constraints.

Accepted Answer

Configure limits at runtime using flags: `--memory="512m"` restricts memory, and `--cpus="2.0"` restricts execution to a maximum of 2 CPU cores. If a container exceeds memory limits, the OS kernel kills it with an Out-of-Memory (OOM) error.

Question 20

Explain Docker networking: overlay networks and bridge networks in multi-host setups.

Accepted Answer

- Bridge networks: Scope to a single host machine, allowing containers on that host to communicate.
- Overlay networks: Connect containers across different host machines dynamically, which is crucial for orchestrations like Swarm or Kubernetes.

Question 21

How do you write tests that run inside Docker containers during CI/CD builds?

Accepted Answer

Configure Dockerfiles with a test target stage. In CI/CD pipelines, run: `docker build --target test-stage .`. This executes compilation and testing checks (like running Jest/JUnit), failing the build if tests fail.

Question 22

Explain the difference between virtual machines and containers in resource allocation.

Accepted Answer

Virtual Machines allocate fixed slices of memory and CPU to guest OSs, which is slow to start and wasteful. Containers share the host kernel and allocate resources dynamically, which is faster and handles dense packing.

Question 23

Explain how to optimize Docker build caching by ordering Dockerfile instructions.

Accepted Answer

Docker caches build steps. If an instruction's dependencies have not changed, it uses the cache. Optimize by copying package manifests and running installations before copying the rest of the source code:
```dockerfile
COPY package.json .
RUN npm install
COPY . .
```
This avoids re-running installations on every code edit.

Question 24

How do you profile container CPU and memory usage using docker stats?

Accepted Answer

Run `docker stats`. This displays a real-time stream of CPU percentage, memory usage, limit capacities, network I/O, and block I/O statistics for all running containers, helping identify performance regressions.

Question 25

How do you mock container dependencies inside Docker Compose environments during integration testing?

Accepted Answer

Define dummy mock service containers in `docker-compose.yml` (like using wiremock or mock-database images). Link application containers to these mock services to run tests without external APIs.

Question 26

Explain how Docker uses Linux namespaces and cgroups.

Accepted Answer

- Namespaces: Provide process isolation (PID, NET, MNT, IPC). Each container sees only its own processes and network interfaces.
- Control Groups (cgroups): Restrict resource allocation (CPU, memory, disk I/O limits) to processes.

Question 27

What is image layer caching and how does it affect registry download speeds?

Accepted Answer

Docker images are built as stacks of read-only layers. When pulling an image, the registry only downloads layers that are missing locally. Minimizing layer modifications speeds up deployments.

Question 28

Explain the difference between host and bridge networking in terms of latency.

Accepted Answer

- `bridge` network routing passes through virtual ethernet links and network address translation (NAT), adding minor latency.
- `host` networking bypasses virtualization, executing network operations at host speeds.

Question 29

What is Docker Swarm and how does it manage services?

Accepted Answer

Docker Swarm is a built-in container orchestration tool. It coordinates groups of Docker hosts (managers and workers), manages service scaling, handles rolling updates, and provides basic load balancing.

Question 30

How do you test container startup health using Healthcheck instructions?

Accepted Answer

Add `HEALTHCHECK` to the Dockerfile: `HEALTHCHECK --interval=30s CMD curl -f http://localhost/health || exit 1`. This instructs Docker to audit container health, reporting states as healthy or unhealthy.

Question 31

Explain how to mount SSH credentials inside Docker builds securely.

Accepted Answer

Use build secrets in Docker: `RUN --mount=type=secret,id=ssh_key ...`. This mounts credentials dynamically during compile stages, keeping secrets out of the final image layers.

Question 32

What is storage driver selection in Docker?

Accepted Answer

Storage drivers (like `overlay2`, `btrfs`, `zfs`) manage how image layers and container writable layers are stored on disk, affecting file read/write speeds.

Question 33

How do you manage container logging drivers in production?

Accepted Answer

By default, Docker saves logs as JSON files, which can consume disk space. Configure logging drivers (like `syslog`, `fluentd`, or log-rotation properties) to manage logs and prevent disk exhaustion.

Question 34

Explain how to secure Docker containers, focusing on Rootless mode, User Namespace remapping, and Seccomp profiles.

Accepted Answer

By default, Docker containers run as the root user. If a container is compromised, the attacker can exploit kernel vulnerabilities to escape to the host. Secure containers by:
1. Rootless Mode: Run the Docker daemon itself as a non-root user, preventing host privilege escalation.
2. User Namespace Remapping: Map the container's root user (UID 0) to a high, unprivileged UID on the host (e.g., UID 1000000).
3. Seccomp Profiles: Apply custom Seccomp (Secure Computing Mode) profiles to restrict kernel system calls (e.g. blocking calls like `chmod` or `ptrace`), reducing kernel exploit surfaces.

Question 35

How do you optimize container base images for enterprise-scale deployments (distroless, scratch, alpine)?

Accepted Answer

To optimize and secure images at scale, replace heavy base images (like `ubuntu` or `node:20` which contain shells and package managers):
- Alpine: A lightweight (5MB) image based on musl libc and busybox, which is small but can cause compatibility issues with C extensions.
- Distroless: Contains only the application and its runtime dependencies (no shell, package manager, or standard utilities), reducing vulnerability scans to near zero.
- Scratch: An empty image used to host statically compiled binaries (like Go/Rust), resulting in tiny, secure images.

Question 36

Explain the architecture of Docker storage drivers (Overlay2) and file copy-on-write (CoW) latency.

Accepted Answer

The `overlay2` driver merges image layers into a single directory structure. When a container reads a file, it reads directly from the read-only image layer. When it writes to a file, the driver copies the file from the image layer to the container's writable layer first (Copy-on-Write). For large files, this copy operation adds latency. Optimize by storing database files inside Docker Volumes, which bypasses the overlay storage driver entirely.

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