build(docker): add an explaining comment to the added dependencies

possible solutions: download a pre-compiled binary; build in a pre-stage; replace icu with another lib

Dockerfile

@@ -15,8 +15,11 @@ LABEL \
     org.label-schema.vcs-url="https://gitlab.gwdg.de/dariah-de/textgridrep/repdav" \
     org.label-schema.vendor="SUB/FE"
 
-RUN apk add --no-cache g++=10.3.1_git20210424-r2 icu-dev=67.1-r2 && \
-    adduser -D repdav
+RUN adduser -D repdav \
+  && apk add --no-cache \
+  # currently needed by icu library; needs to be replaced tomehow...
+  g++=10.3.1_git20210424-r2 icu-dev=67.1-r2
+
 USER repdav
 
 COPY --chown=repdav requirements.txt /