GH Action: Change logic for finding out latest release

Not newest git tag, but latest release from GitHub.
Matches the one installed in the docker image using "go install ..@latest"

.github/workflows/docker-image.yml

@@ -15,14 +15,6 @@ jobs:
     steps:
       - name: Checkout image repo
         uses: actions/checkout@v3
-      - name: Checkout upstream repo
-        uses: actions/checkout@v3
-        with:
-          repository: mailhog/mailhog
-          token: ${{ secrets.GITHUB_TOKEN }}
-          path: upstream
-          # We're only interested in the tags, so check them out, too (not just a single commit)
-          fetch-depth: 0
       - name: Login to GitHub Container Registry
         uses: docker/login-action@v2
         with:
@@ -32,9 +24,8 @@ jobs:
       - name: Create tags
         id: create-tag
         run: |
-          # Find latest tag
-          cd upstream
-          VERSION="$(git describe --tags $(git rev-list --tags --max-count=1))"
+          # Find latest release (same as "go install github.com/mailhog/MailHog@latest" in Dockerfile)
+          VERSION="$( curl -s -H "Authorization: bearer ${{ secrets.GITHUB_TOKEN }}"  "https://api.github.com/repos/mailhog/mailhog/releases/latest" | jq -r '.tag_name')"
           VERSION_DATE="$VERSION-$(date +'%Y-%m-%d')"
           
           GHCR_IMAGE=ghcr.io/${{ github.repository_owner }}/mailhog