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/containers/ocicrypt/reader.go

@@ -0,0 +1,40 @@
+/*
+   Copyright The ocicrypt Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package ocicrypt
+
+import (
+	"io"
+)
+
+type readerAtReader struct {
+	r   io.ReaderAt
+	off int64
+}
+
+// ReaderFromReaderAt takes an io.ReaderAt and returns an io.Reader
+func ReaderFromReaderAt(r io.ReaderAt) io.Reader {
+	return &readerAtReader{
+		r:   r,
+		off: 0,
+	}
+}
+
+func (rar *readerAtReader) Read(p []byte) (n int, err error) {
+	n, err = rar.r.ReadAt(p, rar.off)
+	rar.off += int64(n)
+	return n, err
+}