feat: add support for building arbitrary releases

This will let us support creating a build release that can be used to test anyone's PR.

contrib/update-build.Dockerfile

@@ -13,7 +13,4 @@ COPY . ${WORKDIR}
 
 RUN make deb-dokku-update rpm-dokku-update
 
-RUN mkdir -p /data \
-    && cp /tmp/*.deb /data \
-    && cp /tmp/*.rpm /data \
-    && ls -lha /data/
+RUN ls -lha /tmp/