bib: add cross arch building

This commit adds experimental cross arch building.

I.e. you `bootc-image-builder` on a mac and target an `amd64` machine.

The usage would be:
```
sudo podman run ... \
  --type qcow2 \
  --target-arch amd64 \
     quay.io/centos-bootc/fedora-bootc:eln
```
and this is predicated on the idea that the target container is
available for both host and target architecture. When that is the case
the container for the host architecture is used as a buildroot, the
container for the target architecture is the target tree. Note that some
stages chroot into the target tree so `qemu-user` is essential for this
to work so that target achitecture code can be run transparently (because
only very small amounts of code need to run emulated this is pretty fast
still).

It will need qemu-user{,-static} installed on the host. Sadly we
cannot just install qemu-user into the container because the binfmt_misc
system is global and not namespaced. So while adding the binfmt_misc
handler from inside the container would work with enough privs it
would alter the global machine state and we should not do it.

bib/cmd/bootc-image-builder/main.go

@@ -115,9 +115,19 @@ func makeManifest(c *ManifestConfig, cacheRoot string) (manifest.OSBuildManifest
 	}
 
 	// resolve container
-	resolver := container.NewResolver(c.Architecture.String())
+	hostArch := arch.Current()
+	resolverNative := container.NewResolver(hostArch.String())
+	resolverTarget := container.NewResolver(c.Architecture.String())
+
 	containerSpecs := make(map[string][]container.Spec)
 	for plName, sourceSpecs := range manifest.GetContainerSourceSpecs() {
+		var resolver container.Resolver
+		if plName == "build" {
+			resolver = resolverNative
+		} else {
+			resolver = resolverTarget
+		}
+
 		for _, c := range sourceSpecs {
 			resolver.Add(c)
 		}
@@ -152,8 +162,8 @@ func saveManifest(ms manifest.OSBuildManifest, fpath string) error {
 }
 
 func manifestFromCobra(cmd *cobra.Command, args []string) ([]byte, error) {
-	hostArch := arch.Current()
-	repos, err := loadRepos(hostArch.String())
+	buildArch := arch.Current()
+	repos, err := loadRepos(buildArch.String())
 	if err != nil {
 		return nil, err
 	}
@@ -163,6 +173,20 @@ func manifestFromCobra(cmd *cobra.Command, args []string) ([]byte, error) {
 	configFile, _ := cmd.Flags().GetString("config")
 	tlsVerify, _ := cmd.Flags().GetBool("tls-verify")
 	imgType, _ := cmd.Flags().GetString("type")
+	targetArch, _ := cmd.Flags().GetString("target-arch")
+	if targetArch != "" {
+		// TODO: detect if binfmt_misc for target arch is
+		// available, e.g. by mounting the binfmt_misc fs into
+		// the container and inspects the files or by
+		// including tiny statically linked target-arch
+		// binaries inside our bib container
+		fmt.Fprintf(os.Stderr, "WARNING: target-arch is experimental and needs an installed 'qemu-user' package\n")
+		if imgType == "iso" {
+			return nil, fmt.Errorf("cannot build iso for different target arches yet")
+		}
+		buildArch = arch.FromString(targetArch)
+	}
+	// TODO: add "target-variant", see https://github.com/osbuild/bootc-image-builder/pull/139/files#r1467591868
 
 	var config *BuildConfig
 	if configFile != "" {
@@ -179,7 +203,7 @@ func manifestFromCobra(cmd *cobra.Command, args []string) ([]byte, error) {
 		ImgType:      imgType,
 		Config:       config,
 		Repos:        repos,
-		Architecture: hostArch,
+		Architecture: buildArch,
 		TLSVerify:    tlsVerify,
 	}
 	return makeManifest(manifestConfig, rpmCacheRoot)
@@ -315,6 +339,7 @@ func run() error {
 	manifestCmd.Flags().String("config", "", "build config file")
 	manifestCmd.Flags().String("type", "qcow2", "image type to build [qcow2, ami]")
 	manifestCmd.Flags().Bool("tls-verify", true, "require HTTPS and verify certificates when contacting registries")
+	manifestCmd.Flags().String("target-arch", "", "build for the given target architecture (experimental)")
 
 	logrus.SetLevel(logrus.ErrorLevel)
 	buildCmd.Flags().AddFlagSet(manifestCmd.Flags())