hack: use buildx to cross-build images with docker

When Docker is used and the target architecture differs from the host
architecture, use buildx for cross-building.

Signed-off-by: Alexander Bachmann <[email protected]>

hack/build-image

@@ -125,6 +125,7 @@ QUAL_FQIN = 'fqin'
 
 
 _DISCOVERED_CONTAINER_ENGINES = []
+_BUILDX_BUILDER_NAME = "samba-in-kubernetes"
 
 
 def check_kind(kind):
@@ -203,14 +204,25 @@ def container_engine(cli):
 
 def container_build(cli, target):
     """Construct and execute a command to build the target container image."""
-    args = [container_engine(cli), "build"]
-    pkgs_from = PACKAGES_FROM[target.pkg_source]
-    if pkgs_from:
-        args.append(f"--build-arg=INSTALL_PACKAGES_FROM={pkgs_from}")
-    # docker doesn't currently support alt. architectures
+    eng = container_engine(cli)
+    args = [eng, "build"]
+
     if "docker" in args[0]:
+        # if the target arch and the host_arch are not the same, we need to use buildx
         if target.arch != host_arch():
-            raise RuntimeError("Docker does not support --arch")
+            args = [
+                eng,
+                "buildx",
+                "build",
+                f"--builder={_BUILDX_BUILDER_NAME}",
+                f"--platform=linux/{target.arch}",
+                "--load"
+            ]
+            # Docker's default builder only supports the host architecture.
+            # Therefore, we need to create a new builder to support other
+            # architectures. Errors are suppressed to prevent issues when
+            # the builder is already available - this can be improved later.
+            run(cli, [eng, "buildx", "create", f"--name={_BUILDX_BUILDER_NAME}"], check=False)
     elif target.arch != host_arch() or FORCE_ARCH_FLAG:
         # We've noticed a few small quirks when using podman with the --arch
         # option. The main issue is that building the client image works
@@ -219,6 +231,11 @@ def container_build(cli, target):
         # --arch is not provided. So if the target arch and the host_arch
         # are the same, skip passing the extra argument.
         args.append(f"--arch={target.arch}")
+
+    pkgs_from = PACKAGES_FROM[target.pkg_source]
+    if pkgs_from:
+        args.append(f"--build-arg=INSTALL_PACKAGES_FROM={pkgs_from}")
+
     if cli.extra_build_arg:
         args.extend(cli.extra_build_arg)
     for tname in target.all_names(baseless=cli.without_repo_bases):