Fix Docker builds for git worktrees by mounting main .git directory

When building from a git worktree, the .git file contains a path to the actual
.git directory. The Docker container needs access to the main .git directory
to properly resolve git information and update submodules.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: andrewleech

src/mpbuild/build.py

@@ -14,6 +14,38 @@
 from . import board_database, find_mpy_root
 from .board_database import Board
 
+
+def get_git_directory(mpy_dir: Path) -> Path | None:
+    """
+    Get the actual .git directory path, handling both regular repos and worktrees.
+
+    Args:
+        mpy_dir: Path to the MicroPython repository root
+
+    Returns:
+        Path to the actual .git directory, or None if not found
+    """
+    git_path = mpy_dir / ".git"
+
+    if not git_path.exists():
+        return None
+
+    if git_path.is_dir():
+        # Regular git repository
+        return git_path
+    elif git_path.is_file():
+        # Git worktree - .git file contains path to actual .git directory
+        try:
+            git_file_content = git_path.read_text().strip()
+            if git_file_content.startswith("gitdir: "):
+                gitdir_path = git_file_content[8:]  # Remove "gitdir: " prefix
+                return Path(gitdir_path).resolve()
+        except (OSError, ValueError):
+            pass
+
+    return None
+
+
 ARM_BUILD_CONTAINER = "micropython/build-micropython-arm"
 BUILD_CONTAINERS = {
     "stm32": ARM_BUILD_CONTAINER,
@@ -107,6 +139,16 @@ def docker_build_cmd(
 
     mpy_dir = str(port.directory_repo)
 
+    # Handle git worktrees by mounting the main .git directory
+    git_volume_mount = ""
+    git_dir = get_git_directory(Path(mpy_dir))
+    if git_dir and not git_dir.is_relative_to(Path(mpy_dir)):
+        # For worktrees, mount the main .git directory (parent of worktrees/)
+        main_git_dir = (
+            git_dir.parent.parent
+        )  # /path/to/.git/worktrees/name -> /path/to/.git
+        git_volume_mount = f"-v {main_git_dir}:{main_git_dir} "
+
     # Dynamically find all ttyACM and ttyUSB devices
     tty_devices = []
     for pattern in ["/dev/ttyACM*", "/dev/ttyUSB*"]:
@@ -125,6 +167,7 @@ def docker_build_cmd(
         f"{'-it ' if docker_interactive else ''}"
         f"{device_flags}"                       # provides access to USB and serial devices for deploy
         f"-v {mpy_dir}:{mpy_dir} -w {mpy_dir} " # mount micropython dir with same path so elf/map paths match host
+        f"{git_volume_mount}"                   # mount actual .git directory for worktrees
         f"--user {uid}:{gid} "                  # match running user id so generated files aren't owned by root
         f"-e HOME=/tmp "                        # set HOME to /tmp for container
         f"{build_container} "