Add CI workflows for testing Linux (#3)

Author: liamnichols

.github/workflows/ci.yml

@@ -28,3 +28,17 @@ jobs:
         uses: actions/checkout@v3
       - name: Run Tests
         run: swift test
+  linux:
+    name: Linux (Swift ${{ matrix.swift }})
+    strategy:
+      fail-fast: false
+      matrix:
+        swift: ["5.5", "5.6"]
+    runs-on: ubuntu-latest
+    container: swift:${{ matrix.swift }}
+    steps:
+      - name: Checkout Repo
+        uses: actions/checkout@v3
+      - name: Run Tests
+        run: swift test
+