github: add job for 32 bit Linux

Some warnings may only be generated on 32 bit systems. Add a new job to
compile on a 32 bit Linux system to more readily detect such issues.

Note that we're not using Ubuntu here, but Debian. Ubuntu does not
receive updates for its 32 bit Docker image anymore.

Author: pks-t

.github/workflows/ci.yml

@@ -20,6 +20,12 @@ jobs:
             env:
               CC: "clang"
               CFLAGS: "-Werror -Wall -Wextra -fsanitize=leak"
+          - os: ubuntu-latest
+            generator: Unix Makefiles
+            image: i386/debian:latest
+            env:
+              CC: "clang"
+              CFLAGS: "-Werror -Wall -Wextra"
           - os: macos-latest
             generator: Unix Makefiles
             env:
@@ -37,12 +43,16 @@ jobs:
       fail-fast: false
 
     runs-on: ${{ matrix.platform.os }}
+    container: ${{matrix.platform.image}}
 
     env:
       CC: ${{matrix.platform.env.CC}}
       CFLAGS: ${{matrix.platform.env.CFLAGS}}
 
     steps:
+    - name: Prepare 32 bit container image
+      if: matrix.platform.image == 'i386/debian:latest'
+      run: apt -q update && apt -q -y install cmake gcc libc6-amd64 lib64stdc++6 make python3
     - name: Check out
       uses: actions/checkout@v5
     - name: Build