add semgrep actions

.github/workflows/semgrep.yml

@@ -0,0 +1,38 @@
+# Name of this GitHub Actions workflow.
+name: Semgrep
+
+on:
+  # Scan changed files in PRs (diff-aware scanning):
+  pull_request: {}
+  # Scan on-demand through GitHub Actions interface:
+  workflow_dispatch: {}
+  # Scan mainline branches and report all findings: 
+  push:
+    branches: ["main"]
+  # Schedule the CI job (this method uses cron syntax):
+  schedule:
+    - cron: '20 17 * * *' # Sets Semgrep to scan every day at 17:20 UTC.
+    # It is recommended to change the schedule to a random time.
+
+jobs:
+  semgrep:
+    # User-definable name of this GitHub Actions job:
+    name: semgrep/ci
+    # If you are self-hosting, change the following `runs-on` value: 
+    runs-on: ubuntu-latest
+
+    container:
+      # A Docker image with Semgrep installed. Do not change this.
+      image: returntocorp/semgrep
+
+    # Skip any PR created by dependabot to avoid permission issues:
+    if: (github.actor != 'dependabot[bot]')
+
+    steps:
+      # Fetch project source with GitHub Actions Checkout.
+      - uses: actions/checkout@v3
+      # Run the "semgrep ci" command on the command line of the docker image.
+      - run: semgrep ci
+        env:
+           # Add the rules that Semgrep uses by setting the SEMGREP_RULES environment variable. 
+           SEMGREP_RULES: "p/default r/gitlab" # more at semgrep.dev/explore