Add CI debugging

.github/workflows/main.yaml

@@ -1,6 +1,13 @@
 name: CI
-on: [push, pull_request]
-
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+    inputs:
+      debug_enabled:
+        description: 'Run the build with tmate debugging enabled (https://github.com/marketplace/actions/debugging-with-tmate)'
+        required: false
+        default: false
 
 jobs:
 
@@ -65,6 +72,11 @@ jobs:
       - name: Setup test environment
         run: source ./ci-prepare-bionic.sh ./compilers VARS false || true
 
+        # Enable tmate debugging of manually-triggered workflows if the input option was provided
+      - name: Setup tmate session
+        uses: mxschmitt/action-tmate@v3
+        if: ${{ github.event_name == 'workflow_dispatch' && github.event.inputs.debug_enabled }}
+
       - name: Test compile gcc     @ CMake 3.13
         if: ${{ ! cancelled() }}
         run: ./ci-test-compile.sh ./build gcc all ${{ env.CMAKE_3_13_BIN }}