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/ulikunitz/xz/bits.go

@@ -0,0 +1,79 @@
+// Copyright 2014-2021 Ulrich Kunitz. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xz
+
+import (
+	"errors"
+	"io"
+)
+
+// putUint32LE puts the little-endian representation of x into the first
+// four bytes of p.
+func putUint32LE(p []byte, x uint32) {
+	p[0] = byte(x)
+	p[1] = byte(x >> 8)
+	p[2] = byte(x >> 16)
+	p[3] = byte(x >> 24)
+}
+
+// putUint64LE puts the little-endian representation of x into the first
+// eight bytes of p.
+func putUint64LE(p []byte, x uint64) {
+	p[0] = byte(x)
+	p[1] = byte(x >> 8)
+	p[2] = byte(x >> 16)
+	p[3] = byte(x >> 24)
+	p[4] = byte(x >> 32)
+	p[5] = byte(x >> 40)
+	p[6] = byte(x >> 48)
+	p[7] = byte(x >> 56)
+}
+
+// uint32LE converts a little endian representation to an uint32 value.
+func uint32LE(p []byte) uint32 {
+	return uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 |
+		uint32(p[3])<<24
+}
+
+// putUvarint puts a uvarint representation of x into the byte slice.
+func putUvarint(p []byte, x uint64) int {
+	i := 0
+	for x >= 0x80 {
+		p[i] = byte(x) | 0x80
+		x >>= 7
+		i++
+	}
+	p[i] = byte(x)
+	return i + 1
+}
+
+// errOverflow indicates an overflow of the 64-bit unsigned integer.
+var errOverflowU64 = errors.New("xz: uvarint overflows 64-bit unsigned integer")
+
+// readUvarint reads a uvarint from the given byte reader.
+func readUvarint(r io.ByteReader) (x uint64, n int, err error) {
+	const maxUvarintLen = 10
+
+	var s uint
+	i := 0
+	for {
+		b, err := r.ReadByte()
+		if err != nil {
+			return x, i, err
+		}
+		i++
+		if i > maxUvarintLen {
+			return x, i, errOverflowU64
+		}
+		if b < 0x80 {
+			if i == maxUvarintLen && b > 1 {
+				return x, i, errOverflowU64
+			}
+			return x | uint64(b)<<s, i, nil
+		}
+		x |= uint64(b&0x7f) << s
+		s += 7
+	}
+}