Add bootc CI workflow and Dockerfile

- Create Dockerfile for building bootc with Rust and OSTree support
- Add comprehensive CI workflow based on bootupd-sdboot pattern
- Includes apt-cacher-ng support for faster builds
- Builds bootc from composefs-backend branch
- Creates Debian package with proper dependencies
- Uploads to Forgejo Debian Registry
- Uses docker cp to transfer files from container to host
- Includes all three bootc binaries: bootc, system-reinstall-bootc, bootc-initramfs-setup

Dockerfile

@@ -0,0 +1,24 @@
+# Dockerfile for building bootc
+ARG release_name=unstable
+FROM buildpack-deps:${release_name}
+
+# Install build dependencies
+RUN apt-get update && apt-get install -y \
+    cargo \
+    rustc \
+    pkg-config \
+    libssl-dev \
+    libostree-dev \
+    git \
+    curl \
+    && rm -rf /var/lib/apt/lists/*
+
+# Install Rust toolchain
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+ENV PATH="/root/.cargo/bin:${PATH}"
+
+# Set working directory
+WORKDIR /workspace
+
+# Default command
+CMD ["bash"]