container: add support for uploading to registries

Add a new generic container registry client via a new `container`
package. Use this to create a command line utility as well as a
new upload target for container registries.

The code uses the github.com/containers/* project and packages to
interact with container registires that is also used by skopeo,
podman et al. One if the dependencies is `proglottis/gpgme` that
is using cgo to bind libgpgme, so we have to add the corresponding
devel package to the BuildRequires as well as installing it on CI.

Checks will follow later via an integration test.

vendor/github.com/gorilla/mux/test_helpers.go

@@ -0,0 +1,19 @@
+// Copyright 2012 The Gorilla Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mux
+
+import "net/http"
+
+// SetURLVars sets the URL variables for the given request, to be accessed via
+// mux.Vars for testing route behaviour. Arguments are not modified, a shallow
+// copy is returned.
+//
+// This API should only be used for testing purposes; it provides a way to
+// inject variables into the request context. Alternatively, URL variables
+// can be set by making a route that captures the required variables,
+// starting a server and sending the request to that server.
+func SetURLVars(r *http.Request, val map[string]string) *http.Request {
+	return requestWithVars(r, val)
+}