Use Travis's tag env var for latest calculation

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

hack/gcr-push.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+# Return value is written into LATEST
+LATEST=false
+function is_latest_release() {
+    # If the tag has alpha, beta or rc in it, it's not "latest"
+    if [[ "$TRAVIS_TAG" == *"beta"* || "$TRAVIS_TAG" == *"alpha"* || "$TRAVIS_TAG" == *"rc"* ]]; then
+        LATEST=false
+    else
+        LATEST=true
+    fi
+}
+
+# Always publish for master
+if [ "$BRANCH" == "master" ]; then
+    VERSION="$BRANCH" make all-containers all-push
+fi
+
+# Publish when TRAVIS_TAG is defined.
+if [ ! -z "$TRAVIS_TAG" ]; then
+    # Check if this is the latest release.
+    is_latest_release
+
+    VERSION="$TRAVIS_TAG" TAG_LATEST="$LATEST" make all-containers all-push
+fi