remove buildkit as dependency from the CLI (integrate github.com/moby/buildkit/util/appcontext)

[thaJeztah]

• relates to vendor: update buildkit to v0.12.0, containerd v1.7.2 #4455 (comment)
• relates to integrate frontend/dockerfile/dockerignore from BuildKit moby/patternmatcher#1

This copies the github.com/moby/buildkit/util/appcontext
package as an internal package. The appcontext package from
BuildKit was the only remaining dependency on BuildKit, and
while we may need some of its functionality, the implementation
is not correct for how it's used in docker/cli (so would need
a rewrite).

Moving a copy of the code into the docker/cli (but as internal
package to prevent others from depending on it) is a first step
in that process, and removes the circular dependency between
BuildKit and the CLi.

We are only using these:

tree vendor/github.com/moby/buildkit
vendor/github.com/moby/buildkit
├── AUTHORS
├── LICENSE
└── util
    └── appcontext
        ├── appcontext.go
        ├── appcontext_unix.go
        ├── appcontext_windows.go
        └── register.go

3 directories, 6 files

Before this:

go mod graph | grep ' github.com/docker/cli'
github.com/moby/[email protected] github.com/docker/[email protected]+incompatible

After this:

go mod graph | grep ' github.com/docker/cli'
# (nothing)

But indirect dependencies and circular dependency on moby/moby -> buildkit -> moby/moby and docker/cli can cause "hell".

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Merging #4457 (112d79a) into master (35442a6) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4457   +/-   ##
=======================================
  Coverage   59.70%   59.70%           
=======================================
  Files         288      288           
  Lines       24816    24816           
=======================================
  Hits        14817    14817           
  Misses       9113     9113           
  Partials      886      886           

[thaJeztah]

Quick blurb, because we discussed this in one of the maintainers calls with @tonistiigi @neersighted (and others, but they may recall the exact details);

• the current use of appcontext in the CLI is not correct (also see plugin sigterm handling behavior is odd #4402)
• ... but we may want to keep parts of the behavior
• and.. this is where my memory fails me 🙈 😊 because I don't recall the exact parts we should keep and which parts to remove
• I think the "CTRL-C x3 to force stop" was something that was desirable
• But ... I forgot the other details 🙈

[thaJeztah]

@tonistiigi @neersighted PTAL; this keeps the status-quo on the functionality and does not fix the issues that were discussed about appcontext, but does remove the BuildKit dependency, which I think may be worth considering.

I updated the PR to move the appcontext package to internal (at least until we fixed the issues that were discussed).

[thaJeztah]

Let me bring this one in; we can use #4402 as tracking issue for the remaining work