manifest: use experimentalflags.String("buildroot")

This commit allows to force a container based buildroot as part
of images when setting:
```
$ IMAGE_BUILDER_EXPERIMENTAL=buildroot=ghcr.io/mvo5/fedora-buildroot:41 \
   image-builder manifest --arch=riscv64 minimal-raw --distro=fedora-41
```
and it will replace the buildroot with the given container.

This will allow to do cross-arch building for riscv. The generated
manifest can be build with osbuild (as long as qemu-user is installed)
and will generate a minimal-raw for riscv64.

Cross-building is useful because qemu-user is a lot faster
(and easier to setup) than full riscv system emulation.
A minimal raw build on a really fast AWS machine takes about
100 minutes. A full minimal-raw build on my (moderate) PC takes
about 20min - which is fast enough so that we can run this in
GH workflows as part of the CI.

Long term I would love to see this generalized and that we would
(optionally) support container based buildroots for most/all(?)
our distro/arch combos. This could be done without too much
maintenance burden on our side via e.g. just pointing to the official
 `ubi` containers for rhel that we then install extra rpms into
(like the mkfs.* tools). This would then allow cross-arch image
building for e.g. a raspberry pi from a normal PC. But obviously
this is all long term stuff that needs more design/discussion
(I'm just mentioning it to make clear that this is not a dead-end
but has more potential).

pkg/manifest/build.go

@@ -3,7 +3,9 @@ package manifest
 import (
 	"fmt"
 
+	"github.com/osbuild/images/internal/common"
 	"github.com/osbuild/images/pkg/container"
+	"github.com/osbuild/images/pkg/experimentalflags"
 	"github.com/osbuild/images/pkg/osbuild"
 	"github.com/osbuild/images/pkg/rpmmd"
 	"github.com/osbuild/images/pkg/runner"
@@ -49,6 +51,13 @@ func NewBuild(m *Manifest, runner runner.Runner, repos []rpmmd.RepoConfig, opts
 		opts = &BuildOptions{}
 	}
 
+	// This allows to override the buildroot with a custom container
+	// for e.g. cross-arch-build experiments, remove once we have
+	// a standard way of doing this
+	if fb := experimentalForcedContainerBuildroot(m, runner, opts); fb != nil {
+		return fb
+	}
+
 	name := "build"
 	pipeline := &BuildrootFromPackages{
 		Base:               NewBase(name, nil),
@@ -149,6 +158,25 @@ func (p *BuildrootFromPackages) getSELinuxLabels() map[string]string {
 	return labels
 }
 
+// experimentalForcedContainerBuildroot will return a container buildroot
+// if if the "IMAGE_BUILDER_EXPERIMENTAL=buildroot=<container-ref>" is
+// defined. This allows us to do cross-arch build experimentation.
+func experimentalForcedContainerBuildroot(m *Manifest, runner runner.Runner, opts *BuildOptions) Build {
+	forcedBuildroot := experimentalflags.String("buildroot")
+	if forcedBuildroot == "" {
+		return nil
+	}
+
+	cntSrcs := []container.SourceSpec{
+		{
+			Source:    forcedBuildroot,
+			Name:      forcedBuildroot,
+			TLSVerify: common.ToPtr(false),
+		},
+	}
+	return NewBuildFromContainer(m, runner, cntSrcs, opts)
+}
+
 type BuildrootFromContainer struct {
 	Base
 

pkg/manifest/build_test.go

@@ -6,6 +6,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
+	"github.com/osbuild/images/internal/common"
 	"github.com/osbuild/images/pkg/container"
 	"github.com/osbuild/images/pkg/osbuild"
 	"github.com/osbuild/images/pkg/rpmmd"
@@ -146,3 +147,31 @@ func TestBuildFromContainerSpecsGetSelinuxLabelsWithContainerBuildable(t *testin
 		"/usr/bin/umount": "system_u:object_r:install_exec_t:s0",
 	})
 }
+
+func TestNewBuildWithExperimentalOverride(t *testing.T) {
+	for _, withForcedBuildroot := range []bool{false, true} {
+		if withForcedBuildroot {
+			t.Setenv("IMAGE_BUILDER_EXPERIMENTAL", "buildroot=ghcr.io/ondrejbudai/cool:stuff")
+		} else {
+			t.Setenv("IMAGE_BUILDER_EXPERIMENTAL", "")
+		}
+		mf := New()
+		runner := &runner.Fedora{Version: 42}
+		buildIf := NewBuild(&mf, runner, nil, nil)
+		require.NotNil(t, buildIf)
+		if withForcedBuildroot {
+			br, ok := buildIf.(*BuildrootFromContainer)
+			assert.True(t, ok)
+			assert.Equal(t, []container.SourceSpec{
+				{
+					Source:    "ghcr.io/ondrejbudai/cool:stuff",
+					Name:      "ghcr.io/ondrejbudai/cool:stuff",
+					TLSVerify: common.ToPtr(false),
+				},
+			}, br.containers)
+		} else {
+			_, ok := buildIf.(*BuildrootFromPackages)
+			assert.True(t, ok)
+		}
+	}
+}