In addition, I would recommend to put the exec command before start the app.

CMD ["sh", "-c", "bin/app eval MyApp.Release.migrate && exec bin/app start"]

Without this You’ll not be able to use Ctrl+C to kill the process when you start the app with docker run.

In my case I’ve create a new shell file in my release folder called migrate_and_server with this:

#!/bin/sh
cd -P -- "$(dirname -- "$0")"
./app eval MyApp.Release.migrate && PHX_SERVER=true exec ./app start

Or you can just use this way:

#!/bin/sh
cd -P -- "$(dirname -- "$0")"
./app eval MyApp.Release.migrate && exec ./server

Would be Googler’s heres for Phoenix 1.7.0 and above.

I ran mix phx.gen.release --docker and it generated the Dockerfile for me.

It also generated a migrate and server file.

in rel/overlays/bin/server...

#!/bin/sh
cd -P -- "$(dirname -- "$0")"
PHX_SERVER=true exec ./my_app start

in rel/overlays/bin/migrate...

#!/bin/sh
cd -P -- "$(dirname -- "$0")"
exec ./my_app eval MyApp.Release.migrate

At the end of the generated Dockerfile all I needed to do was:

# BAD: 
CMD ["/app/bin/server"]

# GOOD:
CMD ["sh", "-c", "/app/bin/migrate && /app/bin/server"]

Now if my migrations don’t run the server doesn’t start which is what I want. Hope this helps.

Note that if you do it like this, sh will receive the SIGTERM signal when the docker container is shut down, but it will not pass it to the application, which means after a grace period, a SIGKILL signal is issued and all children will be terminated abruptly. Your application will not be able to shut down gracefully.

It’s better to define a separate entrypoint.sh that runs both commands, or add another overlay as mentioned that runs the migrations before it starts the server.