e2e: make BuildImage parallel-safe

"image rm concurrent" test is still failing, even after containers#18664:

    Error: no contents in "/tmp/podman_test967723851/Dockerfile"

Probable cause: the images are built in parallel, and p.BuildImage()
writes one single Dockerfile. (This almost certainly renders the
test less effective than intended, since the generated images
might end up being identical).

Solution: write and use a uniquely-named Dockerfile

Signed-off-by: Ed Santiago <santiago@redhat.com>

test/e2e/common_test.go

@@ -1188,7 +1188,7 @@ func (s *PodmanSessionIntegration) jq(jqCommand string) (string, error) {
 }
 
 func (p *PodmanTestIntegration) buildImage(dockerfile, imageName string, layers string, label string) string {
-	dockerfilePath := filepath.Join(p.TempDir, "Dockerfile")
+	dockerfilePath := filepath.Join(p.TempDir, "Dockerfile-" + stringid.GenerateRandomID())
 	err := os.WriteFile(dockerfilePath, []byte(dockerfile), 0755)
 	Expect(err).ToNot(HaveOccurred())
 	cmd := []string{"build", "--pull-never", "--layers=" + layers, "--file", dockerfilePath}