Add awareness of mix deps to docker stuff

* Dockerfile now runs mix deps.get before running mix run
* Ignore /app/deps in docker-compose.yml so filesystems are not shared

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

Dockerfile

@@ -25,4 +25,4 @@ WORKDIR /app
 RUN mix local.hex --force
 RUN mix local.rebar --force
 
-CMD mix run --no-halt
+CMD mix deps.get && mix run --no-halt