Run composer install as part of the build process.

.github/workflows/bouncer.yml

@@ -3,6 +3,7 @@ name: Build Nginx + LetsEncrypt Bouncer
 on:
   push:
     paths:
+      - '.github/workflows/bouncer.yml'
       - 'bouncer/**'
   pull_request:
     branches:
@@ -34,6 +35,7 @@ jobs:
           registry: ghcr.io
           username: ${{ secrets.GHCR_USERNAME }}
           password: ${{ secrets.GHCR_PASSWORD }}
+      - uses: php-actions/composer@v6
       - uses: docker/build-push-action@v3
         name: Build & Push
         with: