Add Docker update pipeline/yml, build asset JSON generation command

[dagood]

• Add a build asset JSON generation command.

  • I'm trying to avoid calling this a "manifest" so it's hard to confuse it vs. the dockermanifest model. I call it a "summary" in a couple places.

• Add a yaml pipeline that triggers on microsoft/go build completion and submits an auto-update PR to microsoft/go-docker. If a PR already exists, adds the commit to the end of the PR branch.

  • Adding the commit rather than force-pushing drove a lot of the changes in this PR. I had to make it check for the PR first (rather than after pushing). Once this is incorporated into auto-sync, that'll help with Auto-sync script shouldn't overwrite ongoing fixup work go#68.

• This PR doesn't include much error handling or retries, so updates running at the same time will race. I filed Add error handling/retries to Go -> Docker auto-update infra go#209 to track fixing that up.

• Fix some assumptions about the branch model that I ran into while testing. Mostly, remove assumption that the PR will use a fork.

• Simplify the update commands into just the one for update PRs. It always runs from this repo now, with a temp Git dir to clone go-docker.

---

Here's a testing PR: microsoft/go-images#7
I set up two microsoft/go dev branches to produce build asset JSON files and ran the update build once for each:

• 1.16: https://dev.azure.com/dnceng/internal/_build/results?buildId=1366309&view=results
• 1.15: https://dev.azure.com/dnceng/internal/_build/results?buildId=1366328&view=results

Here's what the dev branches did: https://github.com/microsoft/go/compare/dev/dagood/manifest?expand=1
I used go mod vendor on top of those changes to generate branches that would run in CI, because go-infra is private and I haven't set up auth. I want to set this repo public and use that when I submit the PR to microsoft/go.

[dagood]

@microsoft/golang-compiler PTAL, this is most of the work remaining to get Go build -> Docker updates flowing. Significant chunk of Go util code, here.