This was requested by @artch to help compatibility with the GitHub Package Registry #3245 (comment)

/cc @Phanatic

Looks like additional changes are needed to make this work;

remotes\docker\resolver.go:260:19:warning: r.headers undefined (type *dockerResolver has no field or method headers) (compile) (staticcheck)

Need to backport #2751

OK; I had a look what is needed to get a clean backport, but that would include #3335, which (I think) is a feature.
Perhaps it's ok to add, so I updated this PR as follows:

• revert [release/1.2] Add user agent header to all requests #3338 [release/1.2] Add user agent header to all requests
• backport Explicitly stating utf-8 when fetching oauth token #2871 Explicitly stating utf-8 when fetching oauth token
  • fixes Authorization failure with unicode registry username #2870 Authorization failure with unicode registry username
  • needed to get a clean cherry-pick
• backport Add user agent header to all requests #3335 Add user agent header to all requests
• backport Compute manifest metadata when not provided #3245 Compute manifest metadata when not provided

I'll leave it to the maintainers to decide if this is ok, or if I need to spend some time to adapt the backport to work
without inclusion of #3335

Codecov Report

Merging #3591 into release/1.2 will decrease coverage by 0.06%.
The diff coverage is 36.92%.

@@               Coverage Diff               @@
##           release/1.2    #3591      +/-   ##
===============================================
- Coverage        44.09%   44.03%   -0.07%     
===============================================
  Files              100      100              
  Lines            10790    10837      +47     
===============================================
+ Hits              4758     4772      +14     
- Misses            5293     5325      +32     
- Partials           739      740       +1

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