Build in Debian 10 container

This ensures that we're compatible with Debian's packaged version of
rustc. Ideally we would run the full CI workflow (package build and
installation) on Debian 10, but since that's best done on a full system
and not a container to test systemd packaging etc., leave it alone for
now.

.github/workflows/rust.yml

@@ -13,10 +13,16 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
+    container: debian:10
 
     steps:
     - uses: actions/checkout@v2
+    - name: Install dependencies
+      run: |
+        sudo apt-get update
+        sudo apt-get install build-essential dpkg-dev
+        sudo apt-get build-dep .
     - name: Build
       run: cargo build --verbose
     - name: Run tests
-      run: cargo test --verbose
+      run: cargo test --verbose