docs: remove note about buildkit not supporting git subdirectories

[thaJeztah]

• supersedes / closes Remove note about Git context not supporting subdir #3194
• supersedes / closes Remove note on git:subdir being incompatible with BuildKit #3342

depends on:

• Add support for subdirectories when building from git source moby/buildkit#1684 Add support for subdirectories when building from git source
• git: support subdir component moby/buildkit#2116 git: support subdir component
• Moby vendor update revendor BuildKit (master branch) moby/moby#42473

This has now been implemented in buildkit#2116, so this note can be removed (once integrated into Docker).

- Description for the changelog

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

[thaJeztah]

Hmm.. interesting failure on Jenkins

=== Failed
=== FAIL: e2e/container TestAttachExitCode (0.53s)
    attach_test.go:25: assertion failed: 
        Command:  docker run -d -i --rm registry:5000/alpine:3.6 sh -c read; exit 21
        ExitCode: 125
        Error:    exit status 125
        Stdout:   f4a92a9ab31f5d16421022479140a45c2eb432152c6556a5cc4c571ecdf70fed
        
        Stderr:   docker: Error response from daemon: cgroups: cgroup mountpoint does not exist: unknown.
        
        
        Failures:
        ExitCode was 125 expected 0
        Expected no error

Looks like Jenkins is running docker 19.03; could be that the workers switched to cgroups v2

[thaJeztah]

And a flaky test?

=== Failed
=== FAIL: cli/command/container TestRemoveForce/without_force (0.00s)
Error: Error: No such container: nosuchcontainer
    --- FAIL: TestRemoveForce/without_force (0.00s)
        rm_test.go:37: assertion failed: 
            --- removed
            +++ →
            {[]string}[?->0]:
            	-: <non-existent>
            	+: "mycontainer"
            

[aidan-mundy]

Moby has revendored BuildKit, this likely needs to be merged now

[crazy-max]

LGTM