ci: Proper boolean comparison for SHOULD_PUBLISH

Because `SHOULD_PUBLISH` is an environment variable, it isn't a true
boolean: it's a string. Update the `if:` condition logic to handle it as
a string instead.

.github/workflows/docker.yml

@@ -50,14 +50,14 @@ jobs:
           path: docker/artifacts
 
       - name: Login to Docker Hub
-        if: env.SHOULD_PUBLISH
+        if: env.SHOULD_PUBLISH == 'true'
         uses: docker/login-action@v2
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
       - name: Login to GHCR
-        if: env.SHOULD_PUBLISH
+        if: env.SHOULD_PUBLISH == 'true'
         uses: docker/login-action@v2
         with:
           registry: ghcr.io
@@ -68,7 +68,7 @@ jobs:
         uses: docker/build-push-action@v3
         with:
           context: ./docker
-          push: ${{ env.SHOULD_PUBLISH }}
+          push: ${{ env.SHOULD_PUBLISH == 'true' }}
           no-cache: true
           platforms: linux/arm/v7,linux/arm64,linux/amd64
           tags: ${{ steps.meta.outputs.tags }}