deb: cross-compilation, ephemeral contexts, local builds

Multiple changes:
- New contexts (schroot, unshare)
- Cross-building quirks, with ephemeral contexts and repositories management
- Contexts with parents, global context manager, better lifetime handling
- Local building of binary packages
- Pull: pulling dsc files by default
- Many small bugfixes and changes

Co-authored-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Co-committed-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>

.github/workflows/ci.yml

@@ -12,7 +12,14 @@ env:
 jobs:
   build:
     runs-on: ubuntu-latest
+    container:
+      image: ubuntu:24.04
+      options: --privileged --cap-add SYS_ADMIN --security-opt apparmor:unconfined
     steps:
+    - name: Set up container image
+      run: |
+        apt-get update
+        apt-get install -y nodejs sudo curl wget ca-certificates build-essential
     - uses: actions/checkout@v6
     - uses: dtolnay/rust-toolchain@stable
       with:
@@ -28,7 +35,11 @@ jobs:
     - name: Install runtime system dependencies
       run: |
         sudo apt-get update
-        sudo apt-get install -y pristine-tar sbuild mmdebstrap dpkg-dev
-    - name: Run tests
-      run: cargo test
-  
+        sudo apt-get install -y git pristine-tar sbuild mmdebstrap util-linux dpkg-dev
+    - name: Setup subuid/subgid
+      run: |
+        usermod --add-subuids 100000-200000 --add-subgids 100000-200000 ${USER:-root}
+    - name: Run tests with verbose logging (timeout 30min)
+      env:
+        RUST_LOG: debug
+      run: timeout 30m cargo test -- --nocapture