Fix maintenance GC script
When running the maintenance GC script in docker the following error was
thrown when looking for empty directories:
```
find: unrecognized: -empty
```
Install GNU `findutils` to allow the command to work on alpine.
Change-Id: Ifa5456c167d8770be0a64937a598c9b21d322406
diff --git a/maintenance/git-gc/Dockerfile b/maintenance/git-gc/Dockerfile
index a648021..adb039f 100644
--- a/maintenance/git-gc/Dockerfile
+++ b/maintenance/git-gc/Dockerfile
@@ -6,7 +6,7 @@
COPY scripts/* /usr/bin/scripts/
-RUN apk add --update curl bash git \
+RUN apk add --update curl bash git findutils \
&& curl -o /bin/jgit https://repo.eclipse.org/content/groups/releases//org/eclipse/jgit/org.eclipse.jgit.pgm/${JGIT_VERSION}-r/org.eclipse.jgit.pgm-${JGIT_VERSION}-r.sh \
&& chmod -R +x /bin/jgit /usr/bin/scripts \
&& rm -rf /var/cache/apk/*
