Update login to use token handling code from distribution

[aaronlehmann]

This PR lays the groundwork for a future PR to add OAuth registry authentication (see distribution/distribution#1418 and distribution/distribution#1475). Before this can happen, docker login needs to use the token handling code in docker/distribution instead of using a separate implementation.

The main purpose of this PR is to make docker login use token handling logic defined in docker/distribution, like push and pull already do. However, there's a fair amount of cleanup here as well. The idea is to simplify the login code and clean up the technical debt here so the upcoming PR to add OAuth will be a relatively simple change that can be easily reviewed.

The main changes here are:

• Unify v2 registry "ping" logic between login and push/pull
• Change LookupPullEndpoints to take a hostname instead parsing a full image reference to extract the hostname
• Rename Endpoint to V1Endpoint to make clear it is only used in v1 flows
• Remove getToken function that duplicates token handling code in the github.com/docker/distribution/registry/client/auth auth package. Use the vendored code instead.

This was originally written by @dmcgowan. I am submitting the PR on his behalf (with a few minor edits of my own), since he is away for the next few days.

[icecrime]

Experimental failure looks like a flaky test (cc @thaJeztah: can you please make sure we track that one?)

12:43:59 FAIL: docker_cli_pull_test.go:75: DockerHubPullSuite.TestPullFromCentralRegistryImplicitRefParts
12:43:59 
12:43:59 docker_cli_pull_test.go:106:
12:43:59     c.Assert(out, checker.Contains, "Image is up to date for hello-world:latest")
12:43:59 ... obtained string = "" +
12:43:59 ...     "Using default tag: latest\n" +
12:43:59 ...     "latest: Pulling from library/hello-world\n" +
12:43:59 ...     "03f4658f8b78: Pulling fs layer\n" +
12:43:59 ...     "a3ed95caeb02: Pulling fs layer\n" +
12:43:59 ...     "a3ed95caeb02: Verifying Checksum\n" +
12:43:59 ...     "a3ed95caeb02: Download complete\n" +
12:43:59 ...     "03f4658f8b78: Verifying Checksum\n" +
12:43:59 ...     "03f4658f8b78: Download complete\n" +
12:43:59 ...     "03f4658f8b78: Pull complete\n" +
12:43:59 ...     "a3ed95caeb02: Pull complete\n" +
12:43:59 ...     "Digest: sha256:8be990ef2aeb16dbcb9271ddfe2610fa6658d13f6dfb8bc72074cc1ca36966a7\n" +
12:43:59 ...     "Status: Downloaded newer image for hello-world:latest\n"
12:43:59 ... substring string = "Image is up to date for hello-world:latest"

[aaronlehmann]

Interesting. I saw the same failure on janky in #20831. I don't remember seeing this often in the past. I wonder if something in the tests has changed recently in a way that made this more flaky.

[thaJeztah]

@icecrime I reopened #17214

@aaronlehmann possibly because someone changed the test in #20411 😇 😉

[aaronlehmann]

I think the fix in #20411 was not harmful, but it was insufficient. I'll open a PR that might help.

[thaJeztah]

@aaronlehmann I know, just teasing 👍

[calavera]

LGTM

[RichardScothern]

LGTM 🎉

[thaJeztah]

LGTM