Using Docker to container Phoenix does not persist the dependencies

Hi, I am new to Phoenix and currently trying to get Phoenix into a Docker container.

Somehow, the mix deps.get and mix deps.compile commands used in the Dockerfile are executing, but the results seem not to be persisted, as I am getting this when I use the resulting Image with docker-compose:

-> % docker-compose up                    
Starting api_db_1 ... done
Starting api_web_1 ... 
Starting api_adminer_1 ... done
Attaching to api_db_1, api_web_1, api_adminer_1
db_1       | 2018-01-16 18:09:01.510 UTC [1] LOG:  listening on IPv4 address "0.0.0.0", port 5432
db_1       | 2018-01-16 18:09:01.510 UTC [1] LOG:  listening on IPv6 address "::", port 5432
db_1       | 2018-01-16 18:09:01.519 UTC [1] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
db_1       | 2018-01-16 18:09:01.536 UTC [23] LOG:  database system was shut down at 2018-01-16 18:05:59 UTC
db_1       | 2018-01-16 18:09:01.542 UTC [1] LOG:  database system is ready to accept connections
adminer_1  | PHP 7.2.1 Development Server started at Tue Jan 16 18:09:02 2018
web_1      | Unchecked dependencies for environment dev:
web_1      | * phoenix_live_reload (Hex package)
web_1      |   the dependency is not available, run "mix deps.get"
web_1      | * gettext (Hex package)
web_1      |   the dependency is not available, run "mix deps.get"
web_1      | * cowboy (Hex package)
web_1      |   the dependency is not available, run "mix deps.get"
web_1      | * phoenix_html (Hex package)
web_1      |   the dependency is not available, run "mix deps.get"
web_1      | * phoenix_pubsub (Hex package)
web_1      |   the dependency is not available, run "mix deps.get"
web_1      | * phoenix (Hex package)
web_1      |   the dependency is not available, run "mix deps.get"
web_1      | * postgrex (Hex package)
web_1      |   the dependency is not available, run "mix deps.get"
web_1      | * phoenix_ecto (Hex package)
web_1      |   the dependency is not available, run "mix deps.get"
web_1      | ** (Mix) Can't continue due to errors on dependencies
api_web_1 exited with code 1

When running docker run --rm -it api_web /bin/bash to explore the container, the deps container is also missing in the project directory.

When I execute mix deps.get outside of the container and pass the deps into it per docker-volume it works just fine and the deps directory unsurprisingly appears.

I am taking this docker-image as a base-image: alpine-elixir-phoenix/Dockerfile at master · bitwalker/alpine-elixir-phoenix · GitHub

From there, I do this in my own Dockerfile:

FROM bitwalker/alpine-elixir-phoenix:latest
ENV MIX_ENV=dev
ADD src/mix.exs src/mix.lock ./

RUN mix deps.get
RUN mix do deps.compile, compile, phx.digest
ADD ./src .

CMD ["mix", "phx.server"]

finally, the docker-compose file:

db:
  image: "postgres:latest"
  ports:
    - "5432:5432"
  environment:
    POSTGRES_DB: "api_dev"
    POSTGRES_USER: "postgres"
    POSTGRES_PASSWORD: "postgres"
# phoenix web
web:
  build: "."
  volumes:
    - ./src:/opt/app
  ports:
    - "4000:4000"
  links:
     - "db:db"
  environment:
      MIX_ENV: "dev"
# adminer
adminer:
  image: adminer
  restart: always
  ports:
    - "84:8080"
  links:
     - "db:db"

Has somebody experienced similar problems?
Should I rethink putting the dependencies into the container?