dockerfile: fix default cache IDs

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

frontend/dockerfile/dockerfile2llb/convert_runmount.go

@@ -124,6 +124,9 @@ func dispatchRunMounts(d *dispatchState, c *instructions.RunCommand, sources []*
 			if mount.CacheSharing == instructions.MountSharingLocked {
 				sharing = llb.CacheMountLocked
 			}
+			if mount.CacheID == "" {
+				mount.CacheID = path.Clean(mount.Target)
+			}
 			mountOpts = append(mountOpts, llb.AsPersistentCacheDir(opt.cacheIDNamespace+"/"+mount.CacheID, sharing))
 		}
 		target := mount.Target

frontend/dockerfile/dockerfile_mount_test.go

@@ -21,6 +21,7 @@ var mountTests = []integration.Test{
 	testMountContext,
 	testMountTmpfs,
 	testMountRWCache,
+	testCacheMountDefaultID,
 }
 
 func init() {
@@ -193,3 +194,32 @@ RUN --mount=type=cache,target=/mycache,uid=1001,gid=1002,mode=0751 [ "$(stat -c
 	}, nil)
 	require.NoError(t, err)
 }
+
+func testCacheMountDefaultID(t *testing.T, sb integration.Sandbox) {
+	f := getFrontend(t, sb)
+
+	dockerfile := []byte(`
+FROM busybox
+RUN --mount=type=cache,target=/mycache touch /mycache/foo
+RUN --mount=type=cache,target=/mycache2 [ ! -f /mycache2/foo ]
+RUN --mount=type=cache,target=/mycache [ -f /mycache/foo ]
+`)
+
+	dir, err := tmpdir(
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+	)
+	require.NoError(t, err)
+	defer os.RemoveAll(dir)
+
+	c, err := client.New(context.TODO(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	_, err = f.Solve(context.TODO(), c, client.SolveOpt{
+		LocalDirs: map[string]string{
+			builder.DefaultLocalNameDockerfile: dir,
+			builder.DefaultLocalNameContext:    dir,
+		},
+	}, nil)
+	require.NoError(t, err)
+}