ci: Add build on Ubuntu-18.04 runner

Tests the build for `snitch_cluster`'s Docker container.

.github/workflows/ci.yml

@@ -0,0 +1,25 @@
+# Copyright 2023 ETH Zurich and University of Bologna.
+# Licensed under the Apache License, Version 2.0, see LICENSE for details.
+# SPDX-License-Identifier: Apache-2.0
+
+# Run functional regression checks
+name: snitch
+on: [push, pull_request]
+jobs:
+
+  ####################
+  # Build spike-dasm #
+  ####################
+
+  build-spike-dasm:
+    name: Build spike-dasm
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - name: Build spike
+        run: |
+          autoconf
+          mkdir build
+          cd build
+          ../configure
+          make