Docker Interview Questions for Beginners and Freshers

A platform for packaging an application with its dependencies into a container, so that it runs the same way on any machine with Docker.

An image is a read-only template with the app and its dependencies. A container is a running instance of an image, with its own writable layer. One image can run as many containers.

A virtual machine includes a full operating system, and runs on a hypervisor. A container shares the host's kernel and isolates only the process. Containers are smaller and start in seconds; virtual machines give stronger isolation.

A text file of instructions for building an image, such as FROM, COPY, RUN and CMD. You build it with docker build.

The default public registry, where images are stored and shared. docker pull downloads from it, and docker push uploads to it.

docker run creates and starts a new container. docker start restarts a stopped one. docker exec runs a command inside a running one.

docker ps lists running containers, docker ps -a includes stopped ones, and docker images lists images.

docker logs name, with -f to follow and --tail 50 for the last lines.

docker exec -it name sh, or bash if the image has it.

It publishes a port: traffic to port 8080 on the host goes to port 80 in the container. The host side is on the left.

It runs the container in the background (detached mode).

stop sends SIGTERM, and after a timeout, SIGKILL, so that the app can shut down cleanly. kill sends SIGKILL at once.

RUN runs during the build, and its result is saved in the image. CMD sets the default command that runs when a container starts.

CMD is a default that arguments to docker run replace. ENTRYPOINT sets a fixed program, and arguments are added to it.

COPY copies files. ADD can also unpack local archives and fetch URLs. Prefer COPY, because it is predictable.

Data in the container's writable layer is lost. To keep it, use a volume or a bind mount.

Docker manages volumes in its own storage, which suits production data. A bind mount shares a chosen host folder, which suits development.

On a user-defined network, they reach each other by container or service name. localhost inside a container means that container itself.

A tool for defining multi-container apps in a YAML file. docker compose up -d starts everything, and docker compose down removes it.

It does not mean the newest, and it can change at any time, which makes builds unpredictable. Use specific versions.