Add script for pushing container images via Travis

Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

hack/gcr-push.sh

@@ -0,0 +1,37 @@
+#!/bin/sh
+
+# Return value is written into LATEST
+LATEST=""
+function latest_release() {
+# Loop through the tags since pre-release versions come before the actual versions.
+# Iterate til we find the first non-pre-release
+    for t in $(git tag -l --sort=-v:refname);
+    do
+        # If the tag has alpha, beta or rc in it, it's not "latest"
+        if [[ "$t" == *"beta"* || "$t" == *"alpha"* || "$t" == *"rc"* ]]; then
+            continue
+        fi
+        LATEST="$t"
+        break
+    done
+}
+
+
+# Always publish for master
+if [ "$BRANCH" == "master" ]; then
+    VERSION="$BRANCH" make all-containers all-push
+fi
+
+# Publish when TRAVIS_TAG is defined.
+if [ ! -z "$TRAVIS_TAG" ]; then
+    # Calculate the latest release
+    latest_release
+
+    # Default to pre-release
+    TAG_LATEST=false
+    if [[ "$TRAVIS_TAG" == "$LATEST" ]]; then
+        TAG_LATEST=true
+    fi
+
+    VERSION="$TRAVIS_TAG" TAG_LATEST="$TAG_LATEST" make all-containers all-push
+fi