bake(compose): fix unskipped services without build context

[crazy-max]

fixes #1276

while fixing compose consistency check in #1181 a regression was introduced where services without build context were taken into account.

it was not detected because our test TestNoBuildOutOfTreeService was not checking the targets length:

buildx/bake/compose_test.go

Lines 71 to 82 in 611329f

	func TestNoBuildOutOfTreeService(t *testing.T) {
	var dt = []byte(`
	services:
	external:
	image: "verycooldb:1337"
	webapp:
	build: ./db
	`)
	c, err := ParseCompose(dt, nil)
	require.NoError(t, err)
	require.Equal(t, 1, len(c.Groups))
	}

by fixing this issue we now remove the ability to merge compose files without a build context #1172. To fix this we now parse compose files all together so the compose-go library can handle a proper sanity check on its side.

Signed-off-by: CrazyMax [email protected]