Squashed 'release-tools/' changes from 7bc70e52..1748b16b

1748b16b Merge pull request #136 from pohly/go-1.16
ec844ea6 remove travis.yml, Go 1.16
df76aba8 Merge pull request #134 from andyzhangx/add-build-arg
e314a56d add build-arg ARCH for building multi-arch images, e.g. ARG ARCH FROM k8s.gcr.io/build-image/debian-base-${ARCH}:v2.1.3

git-subtree-dir: release-tools
git-subtree-split: 1748b16b488381c34a86ddbee110e9ed523bcb20

README.md

@@ -21,7 +21,11 @@ The expected repository layout is:
    Dockerfile in the root when only building a single command
  - `Makefile` - includes `release-tools/build.make` and sets
    configuration variables
- - `.travis.yml` - a symlink to `release-tools/.travis.yml`
+ - `.prow.sh` script which imports `release-tools/prow.sh`
+   and may contain further customization
+ - `.cloudbuild.sh` and `cloudbuild.yaml` as symlinks to
+   the corresponding files in `release-tools` or (if necessary)
+   as custom files
 
 To create a release, tag a certain revision with a name that
 starts with `v`, for example `v1.0.0`, then `make push`