Improve buildkit node creation

[silvin-lubecki]

What I did
Move builder and nodes initialization code up, avoiding to recreate/load them for every service build.
Added support of BUILDX_BUILDER env var.

Related issue

(not mandatory) A picture of a cute animal, if possible in relation to what you did

[codecov]

Codecov Report

Patch coverage: 86.95% and project coverage change: +1.63% 🎉

Comparison is base (8a1bf5d) 58.30% compared to head (092cd69) 59.94%.
Report is 1 commits behind head on v2.

❗ Current head 092cd69 differs from pull request most recent head e1171c6. Consider uploading reports for the commit e1171c6 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##               v2   #10843      +/-   ##
==========================================
+ Coverage   58.30%   59.94%   +1.63%     
==========================================
  Files         119      118       -1     
  Lines       10331    10076     -255     
==========================================
+ Hits         6024     6040      +16     
+ Misses       3710     3441     -269     
+ Partials      597      595       -2     

Files Changed	Coverage Δ
pkg/compose/build.go	75.29% <83.33%> (+0.29%)	⬆️
pkg/compose/build_buildkit.go	34.04% <100.00%> (-0.58%)	⬇️

... and 6 files with indirect coverage changes

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[milas]

LGTM

Note to Compose team: we should make sure to mention that BUILDX_BUILDER is now supported for up --build as well in the release notes (see also my non-blocking review comment)

[milas]

Hmm, so we already do a look up in cmd/build.go:

compose/cmd/compose/build.go

Lines 58 to 61 in 8318f66

	builderName := opts.builder
	if builderName == "" {
	builderName = os.Getenv("BUILDX_BUILDER")
	}

But that won't apply for up --build (while this will).

@glours Do you think that --builder should become a global flag on Compose, and let the CLI/cmd part handle the env var lookup?

[I don't think we need to resolve this as part of this PR]