Try tagging image as 'latest', too.

When building locally a 'latest' tag is added to each new image. As it
seems, this is done automatically only for tagged releases in GitHub
Actions.
Let's try setting this tag explicitly as it's the default tag for a
image specification when no other is given.

.github/workflows/docker.yml

@@ -62,7 +62,7 @@ jobs:
         uses: docker/build-push-action@v5
         with:
           push: ${{ github.event_name != 'pull_request' }}
-          tags: ${{ steps.meta.outputs.tags }}
+          tags: ${{ steps.meta.outputs.tags }},latest
           labels: ${{ steps.meta.outputs.labels }}
           cache-from: type=gha
           cache-to: type=gha,mode=max

README.md

@@ -37,7 +37,7 @@ This project provides a container based runtime environment for the
 ## Usage
 
 Build and/or run image with [Docker](https://www.docker.com). You can start
-container on pre-built image with `docker run ghcr.io/kieler/elk-live:master`.
+container on pre-built image with `docker run ghcr.io/kieler/elk-live`.
 
 ### Local image build