Update images.yml per instructions

Update image-ations per instructions in https://github.com/calibreapp/image-actions#usage

.github/workflows/images.yml

@@ -1,12 +1,26 @@
-name: Compress images
-on: pull_request
+name: Compress Images
+on:
+  pull_request:
+    # Run Image Actions when JPG, JPEG, PNG or WebP files are added or changed.
+    # See https://help.github.com/en/actions/automating-your-workflow-with-github-actions/workflow-syntax-for-github-actions#onpushpull_requestpaths for reference.
+    paths:
+      - '**.jpg'
+      - '**.jpeg'
+      - '**.png'
+      - '**.webp'
 jobs:
   build:
+    # Only run on Pull Requests within the same repository, and not from forks.
+    if: github.event.pull_request.head.repo.full_name == github.repository
     name: calibreapp/image-actions
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@master
-      - name: calibreapp/image-actions
-        uses: docker://calibreapp/github-image-actions
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      - name: Checkout Repo
+        uses: actions/checkout@v2
+
+      - name: Compress Images
+        uses: calibreapp/image-actions@main
+        with:
+          # The `GITHUB_TOKEN` is automatically generated by GitHub and scoped only to the repository that is currently running the action. By default, the action can’t update Pull Requests initiated from forked repositories.
+          # See https://docs.github.com/en/actions/reference/authentication-in-a-workflow and https://help.github.com/en/articles/virtual-environments-for-github-actions#token-permissions
+          githubToken: ${{ secrets.GITHUB_TOKEN }}