Merge pull request #6697 from dawidd6/docker-group-writable

Dockerfile: make the prefix group writable

Dockerfile

@@ -40,5 +40,5 @@ ENV PATH=/home/linuxbrew/.linuxbrew/bin:/home/linuxbrew/.linuxbrew/sbin:$PATH \
 # Install portable-ruby and tap homebrew/core.
 RUN HOMEBREW_NO_ANALYTICS=1 HOMEBREW_NO_AUTO_UPDATE=1 brew tap homebrew/core \
 	&& chown -R linuxbrew: /home/linuxbrew/.linuxbrew \
-	&& chmod -R o-w /home/linuxbrew/.linuxbrew \
+	&& chmod -R g+w,o-w /home/linuxbrew/.linuxbrew \
 	&& rm -rf ~/.cache