[CALCITE-3043] Add the ability to publish and promote releases using docker

Author: F21

docker.sh

@@ -432,10 +432,17 @@ promote_release(){
 
     set_git_credentials
 
+    # Tag release and push it
     git tag rel/avatica-$TAG_WITHOUT_RC $LATEST_TAG
-
     git push $GITBOX_URL rel/avatica-$TAG_WITHOUT_RC
 
+    # Merge release branch back to master and push
+    [[ $TAG_WITHOUT_RC =~ ([[:digit:]]+\.[[:digit:]]+)\.[[:digit:]]+$ ]]
+    BRANCH_VERSION=${BASH_REMATCH[1]}
+    git checkout master
+    git merge branch-$TAG_WITHOUT_RC --ff-only
+    git push $GITBOX_URL master
+
     svn checkout $RELEASE_REPO /tmp/release
     rm -rf /tmp/release/$PRODUCT-$TAG_WITHOUT_RC
     mkdir -p /tmp/release/$PRODUCT-$TAG_WITHOUT_RC

site/_docs/howto.md

@@ -568,6 +568,14 @@ svn ci
 The old releases will remain available in the
 [release archive](http://archive.apache.org/dist/calcite/).
 
+Merge the release branch back to master and push it:
+
+{% highlight bash %}
+git checkout master
+git merge branch-X.Y --ff-only
+git push origin master
+{% endhighlight %}
+
 ### Publishing a release using docker:
 This assumes that a rc release was tagged and pushed to the git repository.