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/proglottis/gpgme/callbacks.go

@@ -0,0 +1,42 @@
+package gpgme
+
+import (
+	"sync"
+)
+
+var callbacks struct {
+	sync.Mutex
+	m map[uintptr]interface{}
+	c uintptr
+}
+
+func callbackAdd(v interface{}) uintptr {
+	callbacks.Lock()
+	defer callbacks.Unlock()
+	if callbacks.m == nil {
+		callbacks.m = make(map[uintptr]interface{})
+	}
+	callbacks.c++
+	ret := callbacks.c
+	callbacks.m[ret] = v
+	return ret
+}
+
+func callbackLookup(c uintptr) interface{} {
+	callbacks.Lock()
+	defer callbacks.Unlock()
+	ret := callbacks.m[c]
+	if ret == nil {
+		panic("callback pointer not found")
+	}
+	return ret
+}
+
+func callbackDelete(c uintptr) {
+	callbacks.Lock()
+	defer callbacks.Unlock()
+	if callbacks.m[c] == nil {
+		panic("callback pointer not found")
+	}
+	delete(callbacks.m, c)
+}