x/tools/godoc: Dockerize and convert to run on GAE Flex

[andybons]

godoc relies on https://golang.org/doc/go1.10#goroot but GAE Classic is still running Go 1.9.

Convert to use GAE Flex so that we can rid ourselves of these limitations (and deploy godoc from the release-branch.go1.11)

/cc @bradfitz @dmitshur @katiehockman

[bradfitz]

Milestone: Go 1.6 :)

[bradfitz]

Also, because we're using a Go 1.10 x/tools on golang.org now, certain links don't work from the release notes & blog post, like:

https://golang.org/cmd/go/#hdr-Modules__module_versions__and_more

Because the godoc heading rules. This should be a heading:

Modules, module versions, and more

But it renders as a plain paragraph at the moment.

[dmitshur]

Similarly, this statement from https://golang.org/doc/go1.11#godoc is not yet true on https://golang.org/pkg/:

The godoc web server now shows which version of Go introduced new API features.

As a workaround, in the meantime, you can refer to https://tip.golang.org/pkg/. E.g.:

https://tip.golang.org/pkg/os/#UserCacheDir

[steren]

FYI, Go 1.11 on the App Engine standard environment is coming.