pipeline: remove docker image

[contraintuitiv]

it seems as if the pipeline creates a new image every time but does not delete the old one:

[contraintuitiv]

fix until then:

docker images | grep 'ghcr.io/contraintuitiv/wohnalert/wohnalert' | awk '{print $3}' | tail -n +2 | xargs docker rmi --force

this deletes all the images except the last