fix(macos-container): anchor relative Dockerfile path to build context

`container build` resolves -f relative to the current working directory,
not the build context, so builds failed from any cwd other than the repo
root. Anchor a relative Dockerfile to the context before passing it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/macos_container/util.py

@@ -68,6 +68,11 @@ def build_image(ref: str, context: str, *, dockerfile: str = "") -> None:
     _ensure_builder_dns()
     args = [_CONTAINER, "build", "-t", ref, "--dns", dns_server()]
     if dockerfile:
+        # `container build` resolves -f relative to the current working
+        # directory, not the build context. Anchor a relative Dockerfile to
+        # the context so builds work from any cwd.
+        if not os.path.isabs(dockerfile):
+            dockerfile = os.path.join(context, dockerfile)
         args.extend(["-f", dockerfile])
     args.append(context)
     subprocess.run(args, check=True)

tests/unit/test_macos_container_util.py

@@ -73,6 +73,33 @@ resolver #2
         )
         self.assertTrue(run.call_args_list[-1].kwargs["check"])
 
+    def test_build_image_anchors_relative_dockerfile_to_context(self):
+        status = util.subprocess.CompletedProcess(
+            args=[],
+            returncode=0,
+            stdout=(
+                '[{"status":{"state":"running"},'
+                '"configuration":{"dns":{"nameservers":["9.9.9.9"]}}}]'
+            ),
+            stderr="",
+        )
+        with patch.object(util.subprocess, "run", return_value=status) as run, \
+             patch.object(util.os, "environ", {
+                 "BOT_BOTTLE_MACOS_CONTAINER_DNS": "9.9.9.9",
+             }):
+            util.build_image(
+                "bot-bottle-sidecars:latest",
+                "/repo",
+                dockerfile="Dockerfile.sidecars",
+            )
+        self.assertEqual(
+            [
+                "container", "build", "-t", "bot-bottle-sidecars:latest",
+                "--dns", "9.9.9.9", "-f", "/repo/Dockerfile.sidecars", "/repo",
+            ],
+            run.call_args_list[-1].args[0],
+        )
+
     def test_commit_container_execs_tar_and_builds_image(self):
         # stderr is bytes because subprocess.run uses stderr=PIPE without text=True
         completed = util.subprocess.CompletedProcess(