Add CI workflow to check the license file

Whenever one of the recognized license file names are modified in the repository, the workflow runs to check whether the
license can be recognized and whether it is of the expected type.

Author: per1234

.github/workflows/check-license.yml

@@ -0,0 +1,63 @@
+name: Check License
+
+env:
+  EXPECTED_LICENSE_FILENAME: LICENSE.txt
+  # SPDX identifier: https://spdx.org/licenses/
+  EXPECTED_LICENSE_TYPE: AGPL-3.0
+
+# See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows
+on:
+  push:
+    paths:
+      - ".github/workflows/check-license.yml"
+      # See: https://github.com/licensee/licensee/blob/master/docs/what-we-look-at.md#detecting-the-license-file
+      - "[cC][oO][pP][yY][iI][nN][gG]*"
+      - "[cC][oO][pP][yY][rR][iI][gG][hH][tH]*"
+      - "[lL][iI][cC][eE][nN][cCsS][eE]*"
+      - "[oO][fF][lL]*"
+      - "[pP][aA][tT][eE][nN][tT][sS]*"
+  pull_request:
+    paths:
+      - ".github/workflows/check-license.yml"
+      - "[cC][oO][pP][yY][iI][nN][gG]*"
+      - "[cC][oO][pP][yY][rR][iI][gG][hH][tH]*"
+      - "[lL][iI][cC][eE][nN][cCsS][eE]*"
+      - "[oO][fF][lL]*"
+      - "[pP][aA][tT][eE][nN][tT][sS]*"
+  workflow_dispatch:
+  repository_dispatch:
+
+jobs:
+  check-license:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Install Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ruby # Install latest version
+
+      - name: Install licensee
+        run: gem install licensee
+
+      - name: Check license file
+        run: |
+          # See: https://github.com/licensee/licensee
+          LICENSEE_OUTPUT="$(licensee detect --json --confidence=100)"
+
+          DETECTED_LICENSE_FILE="$(echo "$LICENSEE_OUTPUT" | jq .matched_files[0].filename | tr --delete '\r')"
+          echo "Detected license file: $DETECTED_LICENSE_FILE"
+          if [ "$DETECTED_LICENSE_FILE" != "\"$EXPECTED_LICENSE_FILENAME\"" ]; then
+            echo "ERROR: detected license file doesn't match expected: $EXPECTED_LICENSE_FILENAME"
+            exit 1
+          fi
+
+          DETECTED_LICENSE_TYPE="$(echo "$LICENSEE_OUTPUT" | jq .matched_files[0].matched_license | tr --delete '\r')"
+          echo "Detected license type: $DETECTED_LICENSE_TYPE"
+          if [ "$DETECTED_LICENSE_TYPE" != "\"$EXPECTED_LICENSE_TYPE\"" ]; then
+            echo "ERROR: detected license type doesn't match expected $EXPECTED_LICENSE_TYPE"
+            exit 1
+          fi