Change how docker-compose maps volumes

Only a handful of locations are shared with the docker-compose file,
instead of the entire directory, to prevent build files from being
overwritten by root.

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

Dockerfile

@@ -21,7 +21,6 @@ ENV PATH="/root/.cargo/bin:$PATH"
 RUN mkdir /app
 WORKDIR /app
 
-
 RUN mix local.hex --force
 RUN mix local.rebar --force