How to update /etc/hosts file in Docker image during "docker build"

Question

With a more recent version of docker, this could be done with docker-compose and its extra_hosts directive

Add hostname mappings.
Use the same values as the docker run client --add-host parameter (which should already be available for docker 1.8).

extra_hosts:
 - "somehost:162.242.195.82"
 - "otherhost:50.31.209.229"

In short: modify /etc/hosts of your container when running it, instead of when building it.

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With Docker 17.x+, you have a docker build --add-host mentioned below, but, as commented in issue 34078 and in this answer:

The --add-host feature during build is designed to allow overriding a host during build, but not to persist that configuration in the image.

The solutions mentioned do refer the docker-compose I was suggesting above:

• Run an internal DNS; you can set the default DNS server to use in the daemon; that way every container started will automatically use the configured DNS by default

• Use docker compose and provide a docker-compose.yml to your developers.
  The docker compose file allows you to specify all the options that should be used when starting a container, so developers could just docker compose up to start the container with all the options they need to set.

You can not modify the host file in the image using echo in RUN step because docker daemon will maintain the file(/etc/hosts) and its content(hosts entry) when you start a container from the image.

However following can be used to achieve the same:

ENTRYPOINT ["/bin/sh", "-c" , "echo 192.168.254.10   database-server >> /etc/hosts && echo 192.168.239.62   redis-ms-server >> /etc/hosts && exec java -jar ./botblocker.jar " ]

Key to notice here is the use of exec command as docker documentation suggests. Use of exec will make the java command as PID 1 for the container. Docker interrupts will only respond to that.

See https://docs.docker.com/engine/reference/builder/#entrypoint

I think docker recently added the --add-host flag to docker build which is really great.

[Edit] So this feature was updated on 17.04.0-ce

For more detail on how to use docker build with the --add-host flag please visit: https://docs.docker.com/edge/engine/reference/commandline/build/

Since this still comes up as a first answer in Google I'll contribute possible solution.

Command taken from here suprisingly worked for me (Docker 1.13.1, Ubuntu 16.04) :

docker exec -u 0 <container-name> /bin/sh -c "echo '<ip> <name> >> /etc/hosts"