Create docker.exs and docker-entrypoint + round out Dockerfile

At this point, the implementation is completely working and has been
tested running live and federating with other instances.

Dockerfile

@@ -2,7 +2,7 @@ FROM rinpatch/elixir:1.9.0-rc.0-alpine as build
 
 COPY . .
 
-ENV MIX_ENV prod
+ENV MIX_ENV=prod
 
 RUN apk add git gcc g++ musl-dev make &&\
 	echo "import Mix.Config" > config/prod.secret.exs &&\
@@ -15,18 +15,27 @@ RUN mix deps.get --only prod &&\
 
 FROM alpine:latest
 
+ARG HOME=/opt/pleroma
+ARG DATA=/var/lib/pleroma
+
 RUN echo "http://nl.alpinelinux.org/alpine/latest-stable/community" >> /etc/apk/repositories &&\
 	apk update &&\
 	apk add ncurses postgresql-client
 
-RUN adduser --system --shell /bin/false --home /opt/pleroma pleroma &&\
-	mkdir -p /var/lib/pleroma/uploads &&\
-	chown -R pleroma /var/lib/pleroma &&\
-	mkdir -p /var/lib/pleroma/static &&\
-	chown -R pleroma /var/lib/pleroma &&\
+RUN adduser --system --shell /bin/false --home ${HOME} pleroma &&\
+	mkdir -p ${DATA}/uploads &&\
+	mkdir -p ${DATA}/static &&\
+	chown -R pleroma ${DATA} &&\
 	mkdir -p /etc/pleroma &&\
 	chown -R pleroma /etc/pleroma
 
 USER pleroma
 
-COPY --from=build --chown=pleroma:0 /release/ /opt/pleroma/
+COPY --from=build --chown=pleroma:0 /release ${HOME}
+
+COPY ./config/docker.exs /etc/pleroma/config.exs
+COPY ./docker-entrypoint.sh ${HOME}
+
+EXPOSE 4000
+
+ENTRYPOINT ["/opt/pleroma/docker-entrypoint.sh"]