landlock: Grant permission for artefact handling

When downloading artefacts from an image using Docker Buildx, slsactl
requires write access to ~/.docker/buildx. Without this change the command
slsactl download provenance may fail with:

failed to run download: open /home/levi/.docker/buildx/.lock: permission denied

Signed-off-by: Paulo Gomes <[email protected]>

Author: pjbgf

internal/landlock/landlock.go

@@ -39,7 +39,10 @@ func EnforceOrDie() {
 
 	// We can't really restrict the network access at present
 	err = cfg.RestrictPaths(
-		landlock.RWDirs(filepath.Join(home, ".sigstore")), // Sigstore TUF DB.
+		landlock.RWDirs(
+			filepath.Join(home, ".sigstore"),         // Sigstore TUF DB.
+			filepath.Join(home, ".docker", "buildx"), // Image artefacts handling.
+		),
 		landlock.RODirs(
 			"/proc/self",
 		),