check current copyright year in modified files (#4868)

On PR, ensure current year is listed in copyright of modified files.
Runs on Linux GitHub.

.github/workflows/ci.yml

@@ -1,3 +1,10 @@
+# beacon_chain
+# Copyright (c) 2020-2023 Status Research & Development GmbH
+# Licensed and distributed under either of
+#   * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
+#   * Apache v2 license (license terms in the root directory or at https://www.apache.org/licenses/LICENSE-2.0).
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
 name: CI
 on:
   push:
@@ -54,6 +61,31 @@ jobs:
         # with:
         #  submodules: true
 
+      - name: Check copyright year (Linux)
+        if: runner.os == 'Linux'
+        run: |
+          excluded_extensions="md|png"
+
+          base_branch=$(jq -r '.pull_request.base.ref' "$GITHUB_EVENT_PATH")
+          git fetch origin "$base_branch" 2>/dev/null
+          modified_files=$(git diff --name-only --diff-filter=d "origin/$base_branch" HEAD | grep -vE '\.('$excluded_extensions')$')
+
+          current_year=$(date +"%Y")
+          outdated_files=()
+          for file in $modified_files; do
+            if ! grep -qE "Copyright \(c\) .*$current_year Status Research & Development GmbH" "$file"; then
+              outdated_files+=("$file")
+            fi
+          done
+
+          if (( ${#outdated_files[@]} )); then
+            echo "The following files do not have an up-to-date copyright year:"
+            for file in "${outdated_files[@]}"; do
+              echo "- $file"
+            done
+            exit 1
+          fi
+
       - name: MSYS2 (Windows amd64)
         if: runner.os == 'Windows' && matrix.target.cpu == 'amd64'
         uses: msys2/setup-msys2@v2