⚡️(CI) optimize Docker Hub workflow

We use the Docker Hub Workflow to build and push
our images to Docker Hub, but to check if we
have vulnerabilities in our images as well.
When we are just checking for vulnerabilities,
we don't need to do all the builing steps.
This commit optimizes the workflow by only doing the
necessary steps when we are just checking for
vulnerabilities, so during pull requests
without label "preview" we skip the build steps,
and we do not activate QEMU.

CHANGELOG.md

@@ -25,6 +25,7 @@ and this project adheres to
 - 💄(frontend) align colors and logo with ui-kit v2 #1869
 - 🚸(backend) sort user search results by proximity with the active user #1802
 - 🚸(oidc) ignore case when fallback on email #1880
+- ⚡️(CI) optimize Docker Hub workflow #1919
 
 ### Fixed