Create new targets for running in existing containers (GCB).

1. release-in-a-container - Like 'make release' but in a container.
2. cross-in-a-container - Like 'make cross' but in a container.
3. package-tarballs - (NEW) To package tarballs with a docker dependency

'release-in-a-container' is currently only for testing and is not slated to
be used, but may be useful for testing in some scenarios.

'cross-in-a-container' is meant to be run from the kube-cross image alone.
'package-tarballs' is a companion target that runs from a docker image
to package up the tarballs and images (from cross-in-a-container) for a release.

build/release-in-a-container.sh

@@ -0,0 +1,49 @@
+#!/bin/bash
+# Copyright 2017 The Kubernetes Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+# Complete the release with the standard env
+KUBE_ROOT=$(dirname "${BASH_SOURCE}")/..
+
+# Check and error if not "in-a-container"
+if [[ ! -f /.dockerenv ]]; then
+  echo
+  echo "'make release-in-a-container' can only be used from a docker container."
+  echo
+  exit 1
+fi
+
+# Other dependencies: Your container should contain docker
+if ! type -p docker >/dev/null 2>&1; then
+  echo
+  echo "'make release-in-a-container' requires a container with" \
+       "docker installed."
+  echo
+  exit 1
+fi
+
+
+# First run make cross-in-a-container
+make cross-in-a-container
+
+# at the moment only make test is supported.
+if [[ $KUBE_RELEASE_RUN_TESTS =~ ^[yY]$ ]]; then
+  make test
+fi
+
+$KUBE_ROOT/build/package-tarballs.sh