Container: Add ERL_FLAGS to compile elixir on qemu cross-platform

Without this, compiling Elixir on arm64 using QEMU fails with:
 <<"could not call Module.put_attribute/3 because the module ExUnit.DocTest
 is already compiled">>

Solution found in:
  https://elixirforum.com/t/elixir-docker-image-wont-build-for-linux-arm64-v8-using-github-actions/56383/13

.github/container/Dockerfile

@@ -42,6 +42,7 @@ RUN wget -O - https://github.com/elixir-lang/elixir/archive/v$ELIXIR_VSN.tar.gz
         | tar -xzf -
 
 WORKDIR elixir-$ELIXIR_VSN
+ENV ERL_FLAGS="+JPperf true"
 RUN make install clean
 
 RUN mix local.hex --force \