ADD ./src . is actually fine, if you use ADD ./src/mix.exs ./src/mix.lock ./ before.
Anyways, you brought me on the right track! My problem were the
volumes:
- ./src:/opt/app
lines in the docker-compose.yml file. The Dockerfile was fine, but the dependencies got deleted everytime I brought the container up with docker-compose. Now I only have to find a way to be able to edit the code outside of the container, without deleting my dependencies.
Thank you very much for pointing me in the right direction!
