Fix Dockerfile build issue (#8)

I've found that the Dockerfile does not build on my machine, nor does it build on the build server: https://github.com/exercism/erlang-test-runner/actions/runs/813744510

Converting the aliases to lowercase fixes this

Dockerfile

@@ -1,27 +1,27 @@
-FROM hexpm/erlang:22.3.4.12-ubuntu-focal-20200703 as ERLANG
+FROM hexpm/erlang:22.3.4.12-ubuntu-focal-20200703 as erlang
 
 WORKDIR /app
 
-FROM ERLANG as DOWNLOADER
+FROM erlang as downloader
 
 RUN apt update; apt install --yes curl; \
     mkdir -p /tmp/tools; \
     curl -L https://github.com/erlang/rebar3/releases/download/3.14.4/rebar3 > /tmp/tools/rebar3; \
     chmod +x /tmp/tools/*
 
-FROM ERLANG as BUILDER
+FROM erlang as builder
 
-COPY --from=DOWNLOADER /tmp/tools/rebar3 rebar3
+COPY --from=downloader /tmp/tools/rebar3 rebar3
 COPY rebar.config rebar.lock ./
 RUN ./rebar3 get-deps
 
 COPY src/ src/
 RUN ./rebar3 escriptize; find . -type f -executable
 
-FROM ERLANG as RUNNER
+FROM erlang as runner
 
-copy --from=BUILDER /app/_build/default/lib/erl_exercism /opt/erl_exercism
-COPY --from=BUILDER /app/_build/default/bin/erlang_test_runner /opt/test-runner/bin/
+COPY --from=builder /app/_build/default/lib/erl_exercism /opt/erl_exercism
+COPY --from=builder /app/_build/default/bin/erlang_test_runner /opt/test-runner/bin/
 COPY run.sh /opt/test-runner/bin/run.sh
 
 ENV ERL_LIBS=/opt/erl_exercism