osxkeychain: match min macos version for xx

[crazy-max]

partially solves #280

xx sets a minimal macos version if not set through MACOSX_VERSION_MIN env var: https://github.com/tonistiigi/xx/blob/3d00d096c8bf894ec29bae5caa5aea81d9c187a5/base/xx-info#L203-L209

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (0b91805) 55.23% compared to head (5944f8a) 55.23%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #283   +/-   ##
=======================================
  Coverage   55.23%   55.23%           
=======================================
  Files           9        9           
  Lines         668      668           
=======================================
  Hits          369      369           
  Misses        256      256           
  Partials       43       43           

Impacted Files	Coverage Δ
osxkeychain/osxkeychain_darwin.go	69.10% <ø> (ø)

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[thaJeztah]

Haven't tried yet, but wondering if we need to update these two lines;

docker-credential-helpers/osxkeychain/osxkeychain_darwin.go

Lines 4 to 5 in f8e94d9

	#cgo CFLAGS: -x objective-c -mmacosx-version-min=10.11
	#cgo LDFLAGS: -framework Security -framework Foundation -mmacosx-version-min=10.11

[crazy-max]

Haven't tried yet, but wondering if we need to update these two lines;

docker-credential-helpers/osxkeychain/osxkeychain_darwin.go

Lines 4 to 5 in f8e94d9

	#cgo CFLAGS: -x objective-c -mmacosx-version-min=10.11
	#cgo LDFLAGS: -framework Security -framework Foundation -mmacosx-version-min=10.11

I think we can keep 10.11 as minimum version looking at the current implementation of the keychain in our codebase.

[thaJeztah]

LGTM