Since ejabberd 21.07, mix.exs expects that vars.config contains vsn

ecs/Dockerfile

@@ -9,6 +9,7 @@ LABEL maintainer="ProcessOne <contact@process-one.net>" \
 RUN git clone https://github.com/processone/ejabberd.git
 WORKDIR /ejabberd
 COPY vars.config .
+RUN echo '{vsn, "'${VERSION}'.0"}.' >>vars.config
 COPY rel/*exs rel/
 RUN git checkout ${VERSION/latest/HEAD} \
     && mix deps.get \