Add a conformance test for copying to a mounted prior stage

Add a conformance test for cases where an intermediate stage mounts the
contents of a previous stage in a read-write fashion and modifies it.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

tests/conformance/conformance_test.go

@@ -3081,6 +3081,12 @@ var internalTestCases = []testCase{
 		contextDir:        "env/precedence",
 		dockerUseBuildKit: true,
 	},
+
+	{
+		name:              "multistage-copyback",
+		contextDir:        "multistage/copyback",
+		dockerUseBuildKit: true,
+	},
 }
 
 func TestCommit(t *testing.T) {

tests/conformance/testdata/multistage/copyback/Dockerfile

@@ -0,0 +1,6 @@
+FROM alpine AS base
+RUN touch -r /etc/os-release /1.txt
+FROM alpine AS interloper
+RUN --mount=type=bind,from=base,source=/,destination=/base,rw touch -r /etc/os-release /base/2.txt
+FROM base
+RUN --mount=type=bind,from=interloper,source=/etc,destination=/etc2 touch -r /etc2/os-release /3.txt