Install Docker plugin for Packer separately

This is now required, since the latest version of Packer.

.github/workflows/check-image-definitions.yml

@@ -6,6 +6,9 @@ name: Validate Packer images
   - push
   - pull_request
 
+permissions:
+  contents: read
+
 jobs:
   validate:
     runs-on: ubuntu-latest
@@ -17,6 +20,7 @@ jobs:
         run: |
           sudo apt update -y
           sudo apt install -y packer
+          packer plugins install github.com/hashicorp/docker
 
       - name: Validate Packer images
         run: |

.github/workflows/push-docker-image.yml

@@ -1,16 +1,20 @@
+---
 # Build and push the selected docker image from a packer config.
 
 name: Push Docker image
 
 # Controls when the action will run: manual trigger with a parameter
-on:
+'on':
   workflow_dispatch:
     inputs:
       image-name:
         description: Docker image to build
         required: true
         default: slc8-builder
 
+permissions:
+  contents: read
+
 # A workflow run is made up of one or more jobs that can run sequentially or in parallel
 jobs:
   # This workflow contains a single job called "build"
@@ -36,6 +40,7 @@ jobs:
           apt install -y software-properties-common
           add-apt-repository universe
           apt install -y docker.io packer
+          packer plugins install github.com/hashicorp/docker
         env:
           DEBIAN_FRONTEND: noninteractive