bib: addadd 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).

Containerfile

@@ -13,7 +13,7 @@ FROM registry.fedoraproject.org/fedora:39
 # - https://github.com/osbuild/bootc-image-builder/issues/9
 # - https://github.com/osbuild/osbuild/pull/1468
 COPY ./group_osbuild-osbuild-fedora-39.repo /etc/yum.repos.d/
-RUN dnf install -y osbuild osbuild-ostree osbuild-depsolve-dnf podman qemu-img && dnf clean all
+RUN dnf install -y osbuild osbuild-ostree osbuild-depsolve-dnf podman qemu-img qemu-user-static && dnf clean all
 COPY --from=builder /build/bin/bootc-image-builder /usr/bin/bootc-image-builder
 COPY entrypoint.sh /
 

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 +173,14 @@ 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 != "" {
+		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 +197,7 @@ 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 +333,7 @@ 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")
 
 	logrus.SetLevel(logrus.ErrorLevel)
 	buildCmd.Flags().AddFlagSet(manifestCmd.Flags())