fix(docker): Unpack access_token correctly from auth header from docker registries that support it (#5264)

Closes #5260

Co-authored-by: Rhys Arkins <rhys@arkins.net>

Co-authored-by: Jared Kauppila <Jared@Kauppi.la>

Buildkite plugins that don't have required config options may be
expressed as:

```yaml
- plugins:
    - my-plugin#v1.0.0
```

Note that there is no trailing colon; matching on it should be
unnecessary given that the preceding capture groups exclude this
character.

Co-authored-by: Rhys Arkins <rhys@arkins.net>

Ref #4528