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-debian10.yml

@@ -0,0 +1,26 @@
+name: "Debian 10"
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    container: debian:10
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install dependencies
+      run: |
+        apt-get update
+        apt-get install build-essential dpkg-dev
+        apt-get build-dep .
+    - name: Build
+      run: cargo build --verbose