Update Dockerfile

[Chaz6]

What kind of change does this PR introduce?

This change allows the Docker container to build correctly.

What is the current behavior?

STEP 7/10: RUN mix compile
Unchecked dependencies for environment prod:
* libcluster (Hex package)
  the dependency is not available, run "mix deps.get"
* postgrex (Hex package)
  the dependency is not available, run "mix deps.get"
** (Mix) Can't continue due to errors on dependencies

What is the new behavior?

The container is built without failure.

Additional context

N/A