Add a fairly minimal set of files to deploy watchflakes as a service.
Use Cloud Build for building Docker images, so that it's possible to
perform deploys without having Docker as a local dependency.

Also add a README that points to documentation on how to deploy.

For golang/go#65353.

Change-Id: I355a23d02b9a31f99e34b8ab1a1681ec3b9a36a6
Reviewed-on: https://go-review.googlesource.com/c/build/+/579315
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Unfortunately, the only API that creates a github.Client requires that
credentials for api.github.com are available in $HOME/.netrc. It would
be nicer to be able to pass a custom *http.Client. For now, modify the
global default HTTP client (http.DefaultClient) that the rsc.io/github
package always uses and provide authentication to it directly that way,
without needing to write to a $HOME/.netrc file. This can go away when
the rsc.io/github API provides a cleaner way of doing this.

For golang/go#65353.

Change-Id: Ie2ec3e957f7b2dc48b85ce5475a09d19df71622d
Reviewed-on: https://go-review.googlesource.com/c/build/+/579316
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Make it run equally often, no matter how long a single iteration takes.

For golang/go#65353.

Change-Id: I6c2814c1b91e4eef65e09d858b984290a3d5585f
Reviewed-on: https://go-review.googlesource.com/c/build/+/579318
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

Make some progress towards it being possible to handle unexpected errors
by logging them, waiting a bit, and trying again, instead of log.Fatalln
whenever anything unexpected happens, now that watchflakes is becoming a
continuously running service.

Also use some higher level APIs for limited concurrency, and various doc
improvements.

For golang/go#65353.

Change-Id: I7af83a8f368ff41d176d80a475698a04ca407325
Reviewed-on: https://go-review.googlesource.com/c/build/+/579320
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

It turns out to be possible¹ for a build to fail early enough that it
doesn't get a chance to produce output properties, and not get marked
as an infrastructure failure (even though it should've been). There's
not much choice but to ignore such builds.

¹ For example, https://ci.chromium.org/b/8752293940218446529. It failed
  very early while running bbagent, before golangbuild was even started.

For golang/go#65353.

Change-Id: I14782f41e7a390380a32272b58cda385c91cb881
Reviewed-on: https://go-review.googlesource.com/c/build/+/579299
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
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Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

Thanks to Russ for adding it.

For golang/go#65353.

Change-Id: Ie8aa589ade3e076c6a0c1dcf6776dc85a8a3aec2
Reviewed-on: https://go-review.googlesource.com/c/build/+/580016
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>