vendor: update buildkit to master@31c870e82a48

Signed-off-by: Justin Chadwell <me@jedevc.com>

vendor/github.com/moby/buildkit/client/llb/merge.go

@@ -70,6 +70,31 @@ func (m *MergeOp) Inputs() []Output {
 	return m.inputs
 }
 
+// Merge merges multiple states into a single state. This is useful in
+// conjunction with [Diff] to create set of patches which are independent of
+// each other to a base state without affecting the cache of other merged
+// states.
+// As an example, lets say you have a rootfs with the following directories:
+//
+//	/ /bin /etc /opt /tmp
+//
+// Now lets say you want to copy a directory /etc/foo from one state and a
+// binary /bin/bar from another state.
+// [Copy] makes a duplicate of file on top of another directory.
+// Merge creates a directory whose contents is an overlay of 2 states on top of each other.
+//
+// With "Merge" you can do this:
+//
+//	fooState := Diff(rootfs, fooState)
+//	barState := Diff(rootfs, barState)
+//
+// Then merge the results with:
+//
+//	Merge(rootfs, fooDiff, barDiff)
+//
+// The resulting state will have both /etc/foo and /bin/bar, but because Merge
+// was used, changing the contents of "fooDiff" does not require copying
+// "barDiff" again.
 func Merge(inputs []State, opts ...ConstraintsOpt) State {
 	// filter out any scratch inputs, which have no effect when merged
 	var filteredInputs []State