Add composefs CI workflow and Dockerfile

- Create Dockerfile for building composefs with meson/ninja
- Add comprehensive CI workflow based on bootupd-sdboot
- Includes apt-cacher-ng support for faster builds
- Builds composefs from source with FUSE support
- Creates Debian package with proper dependencies
- Uploads to Forgejo Debian Registry
- Uses docker cp to transfer files from container to host

Dockerfile

@@ -0,0 +1,18 @@
+# Dockerfile for building composefs
+ARG release_name=unstable
+FROM buildpack-deps:${release_name}
+
+# Install build dependencies
+RUN apt-get update && apt-get install -y \
+    meson \
+    ninja-build \
+    libfuse3-dev \
+    pkg-config \
+    git \
+    && rm -rf /var/lib/apt/lists/*
+
+# Set working directory
+WORKDIR /workspace
+
+# Default command
+CMD ["bash"]