Fix Dockerfile on elixir 1.11.2

This is a teporary measure aimed at fixing #35
It also adds a CMD as suggested in #22

ops/Dockerfile

@@ -1,4 +1,7 @@
-FROM elixir:1.11-alpine as build
+# FIXME: Loosen Elixir Docker image requirement once 
+# https://github.com/gen-smtp/gen_smtp/issues/250
+# and https://github.com/erlang/otp/issues/4585 have been fixed
+FROM elixir:1.11.2-alpine as build
 ENV MIX_ENV=prod
 
 RUN apk add git npm build-base
@@ -18,11 +21,12 @@ RUN npm run deploy --prefix ./assets && \
     mix phx.digest && \
     mix release
 
-FROM elixir:1.11-alpine
+FROM elixir:1.11.2-alpine
 ENV HOME=/opt/app
 WORKDIR ${HOME}
 COPY --from=build _build/prod/rel/keila ${HOME}
 RUN mkdir -p ${HOME} && \
     adduser -s /bin/sh -u 1001 -G root -h ${HOME} -S -D default && \
     chown -R 1001:0 ${HOME}
 ENTRYPOINT ["/opt/app/bin/keila"]
+CMD "start"