Docker Interview Questions

Docker Image

How you can pull a mysql docker image from docker hub to your local machine ?

docker pull mysql

How you can pull a docker image from private repository in docker hub. The username is test, repository/image name is myimage and the tag is 1.0.0

Since its a private repository, first we have to login to the registry.

docker login -u test -p ***************

Then we can pull the image with the following image name format username/imagename:tag

docker pull test/myimage:1.0.0

What is the image name format to pull a docker image from Jfrog docker registry or any registry apart from docker hub ?

registry-url/registry-name/image-name:tag

Example for pulling a docker image from Jfrog Docker Registry.

username.jfrog.io/default-docker-local/hello-world:tag

Docker Container`

Can you write a command to create a nginx docker container

docker run -d --name nginx nginx:latest

Why ubuntu docker container stops immediately ?

When we create a container, the combination of ENTRYPOINT and CMD from Dockerfile will be executed.

For ubuntu docker container the CMD in Dockerfile is bash command. The bash command is not a background process, it just starts and stop immediately, thats why the ubuntu container stops immediately.

Dockerfile of Ubuntu docker image : https://hub.docker.com/layers/ubuntu/library/ubuntu/latest/images/sha256-3f7c2c6e153e218a10ff78b3868811795fa09cc5d01be28296390ac75ab018b0?context=explore

Docker command to forward specific port

docker run -d --name nginx -p 80:80 nginx:latest

How do you persist some specific files or folder from docker container

By creating the volumes

Example: mysql docker container

docker run --name some-mysql -v /my/own/datadir:/var/lib/mysql -e MYSQL_ROOT_PASSWORD=my-secret-pw -d mysql

How to pass a dockerfile which is in different folder to the docker build command

docker build -f dockerfiles/Dockerfile -t sample .

How you can create and persist the Environment variable in docker container

FROM ubuntu
ENV VERSION=1.0.0

How you can define environment variables to persist only in that particular layer during docker build

FROM ubuntu
RUN export VERSION=1.0.0 && echo $VERSION

You have defined one Environment variable APP_COLOR=red in Dockerfile and while creating the container you can passed arguments -e APP_COLOR=blue. Which value will it take inside a container ?

Environment variable defined during docker run command will be taken.

Environment variable defined during docker run command has the highest priority, when compared to environment variable defined in Dockerfile.

How to reuse the Dockerfile or how to templatise the Dockerfile or How to define the commands commonly in one dockerfile

Using onbuild in base Dockerfile. Which will be executed in the child dockerfile during build time

How namespace and cgroups helps in creating the docker container

What is multistage dockerfile ?

How to write multistage dockerfile

When you will use CMD and ENTRYPOINT ?

In our container, if our executable is not going to change very frequently, we can put excutable in ENTRYPOINT and their arguments in CMD.

And if you want to change only your arguments you can override the CMD during container creation.

If we are not sure about the executable to be used. In that case we can put both executable and their arguments in CMD itself.

During container creation, if we override the CMD, basically we are overriding both executables and arguments.

Official Documentation CMD ENTRYPOINT

You have defined two CMD instructions in Dockerfile, which will be taken ?

The last defined CMD will be considered.

Is is possible to define two ENTRYPOINT in Dockerfile ?

Yes we can define two ENTRYPOINTs in Dockerfile, last ENTRYPOINT will be considered.

It it possible to override ENTRYPOINT while creating container ?

Yes, its possible to override the ENTRYPOINT during container creation.

docker run -it --entrypoint /bin/bash nginx

Have you worked on any docker scanning tools in your CI/CD pipeline ?

Yes, I can worked on docker scanning tools like Anchore, Mcafee, snyk and Jfrog Xray

You want to build a Dockerfile, whether you will keep a Dockerfile in root directory or in some other directory? why?

Its a best practice to not keep a Dockerfile in root directory, because lot of other files will be there in the root directory, while during the docker build all these file contexts will be sent to docker daemon. So the build process will take some time.

But if we need the Dokcerfile in some different folder, only that specific will be sent as context to docker daemon, so the build process will be much faster.

Due to some scenario, if you still want to keep your Dockerfile on root directory. We can use .dockerignore file to ignore the other files and directories that are not required during docker build.

Whats the difference between docker kill and docker stop ?

docker stop The main process inside the container will receive SIGTERM and after a grace period, SIGKILL
Official Documentation

docker kill The main process inside the container is sent SIGKILL signal (default), or the signal that is specified with the –signal option
Official Documentation

docker kill --signal=SIGHUP  my_container

How to clean up unused images, stopped containers and unused networks using single command?

You want to transfer the Docker image from one server to another server, but you have only ssh access. Is it possible ?

Yes, First we have to convert the docker image to tar file and copy the file to another server and convert the tar file to docker image

Command to convert docker image to tar file

docker save busybox > busybox.tar
or
docker save --output busybox.tar busybox
or
docker save -o fedora-latest.tar fedora:latest

official Documentation

Command to convert tar file to docker image

docker load < busybox.tar
or
docker load --input fedora-latest.tar

official Documentation

Can we host windows containers on linux machine?

No. Containers are using the underlying Operating System resources and drivers, so Windows containers can run only on Windows OS, and Linux containers can run only on Linux OS

Whats in Intermediate container ?

During the docker build process, it will create intermediate container for RUN,COPY,ADD instructions in Dockerfile.

The intermediate container will use previous layer as base image, executes the Instructions and then save this container as a layer.

You have couple of docker containers running in a server. You have restarted the server, now you are checking the docker containers, but all the containers are in stopped state. why ?

By default the restart policy for all containers are set to no

If we want to start our docker containers automatically during system reboot, We should set the restart policy to always while creating the container.

docker run -d --restart always nginx

Official Documentation

How you can copy a file to running container without stopping it ?

docker cp hello.txt mycontainer:/hello.txt

Official Documentation

How will you check the live cpu and memory usage of containers ?

docker stats

Official Documentation

Is it possible to rename your container ?

Yest its possible to rename the container

docker rename old_container_name new_container_name