sbt-native-packager docker: How to add entry in /etc/hosts - docker

Question

I'm using the docker plugin of sbt-native-packager to build a Docker image. I would like my image to have an additional entry in /etc/hosts.
I've tried the following:
dockerCommands in Docker := dockerCommands.value.flatMap {
case cmd#Cmd("FROM", _) =>
List(Cmd("FROM", "anapsix/alpine-java")) ++ List(
Cmd("ENV", "JAVA_MIN_MEM", "1024m"),
Cmd("RUN", "echo 8.8.8.8 foo >> /etc/hosts")
)
}
Unfortunately it doesn't seem to work. When I start a container based on this image, the /etc/hosts does not have the extra entry.
It looks like it's actually writing the file because I tried the following instead:
....
Cmd("RUN", "echo 8.8.8.8 foo >> /etc/hosts; ping -c 4 foo")
....
And I'm getting as output the following:
[info] Step 9/15 : RUN echo 8.8.8.8 foo >> /etc/hosts; ping -c 4 foo
[info] ---> Running in b6d7ba25f96f
[info] PING foo (8.8.8.8): 56 data bytes
[info] 64 bytes from 8.8.8.8: seq=0 ttl=37 time=5.521 ms
[info] 64 bytes from 8.8.8.8: seq=1 ttl=37 time=3.188 ms
[info] 64 bytes from 8.8.8.8: seq=2 ttl=37 time=6.012 ms
[info] 64 bytes from 8.8.8.8: seq=3 ttl=37 time=4.192 ms
So it looks like the modified /etc/hosts is being overridden!
What is the correct way to do this?

Answer 1

The file /etc/hosts is managed by Docker and cannot be customized as part of building an image.
As you already figured out, you can add a custom entry using RUN echo 8.8.8.8 foo > /etc/hosts; <some_command_requiring_custom_hosts_file>. But this modification is only available during the execution of this particular RUN command.
In case you need custom entries when running containers use the --add-host parameter of docker run (see docs).
In general it is a best practise to not include configuration details in a Docker images. Applying configuration only at the time you are running your containers helps to keep the images portable.

Answer 2

Yes, Docker overrides that file (in reality, it is a file present in the host system that is mounted in that location when the container starts), so any change you do there will be overriden.
One option would be to change the docker entrypoint to, instead of pointing to your application startup script, point to a script that does that change and then runs your app startup script.
So in plain docker (sorry I have never used sbt docker plugin), instead of having an entry to your app start script (using /usr/bin/myapp):
ENTRYPOINT /usr/bin/myapp
you would have
RUN echo "echo 8.8.8.8 foo > /etc/hosts" >> /startup.sh
RUN echo "/usr/bin/myapp" >> /startup.sh
RUN chmod +x /startup.sh
ENTRYPOINT /startup.sh