Add context to registry push errors

[jonnystoten]

• Extract containerimage export push function
• Wrap push error with target name
• Use original error's message for gRPC error

Fixes #2019.

Registry push error from buildx before this change:

 => ERROR exporting to image                                                                                                                                                                                1.1s
 => => exporting layers                                                                                                                                                                                     0.0s
 => => exporting manifest sha256:d2cfb97f444d76147c440c6b86b333d388735b75b3644718cb5a4e0ad4e23762                                                                                                           0.0s
 => => exporting config sha256:35d75deef09995f6980a80122699b12059b946997dc2acf0d121970cd662c103                                                                                                             0.0s
 => => pushing layers                                                                                                                                                                                       1.1s
------
 > exporting to image:
------
ERROR: failed to solve: server message: insufficient_scope: authorization failed

and after:

 => ERROR exporting to image                                                                                                                                                                                1.4s
 => => exporting layers                                                                                                                                                                                     0.0s
 => => exporting manifest sha256:d2cfb97f444d76147c440c6b86b333d388735b75b3644718cb5a4e0ad4e23762                                                                                                           0.0s
 => => exporting config sha256:35d75deef09995f6980a80122699b12059b946997dc2acf0d121970cd662c103                                                                                                             0.0s
 => => pushing layers                                                                                                                                                                                       1.4s
------
 > exporting to image:
------
ERROR: failed to solve: failed to push docker.io/username/image: server message: insufficient_scope: authorization failed

[tonistiigi]

We don't allow fmt.Errorf in build repos because they lose stacktraces. Use pkg/errors.Wrap

[jonnystoten]

Good to know, thanks!

[aaronlehmann]

Does it lose stack traces even with %w?

[tonistiigi]

Yes. Stdlib errors does not capture a stacktrace.

[aaronlehmann]

Ah, makes sense. I misunderstood your comment and thought you meant it would remove existing stack traces from the error chain.

[tonistiigi]

Could you add a comment in here that "If grpc error was wrapped with additional message then set full message to grpc error".

Otherwise it is quite tricky to understand why this code is in here.