manifest push: handle cert-dir flag

Prior to this commit the value of the `--cert-dir` flag
specified for `podman manifest push` was not handled by the internal
code.

That resulted in `podman manifest push` not reading the certificates
stored inside of the directory specified by the user.

Signed-off-by: Flavio Castelli <fcastelli@suse.com>

pkg/domain/infra/abi/manifest.go

@@ -208,6 +208,7 @@ func (ir *ImageEngine) ManifestPush(ctx context.Context, names []string, opts en
 	}
 	sys.AuthFilePath = opts.Authfile
 	sys.DockerInsecureSkipTLSVerify = opts.SkipTLSVerify
+	sys.DockerCertPath = opts.CertDir
 
 	if opts.Username != "" && opts.Password != "" {
 		sys.DockerAuthConfig = &types.DockerAuthConfig{