Jenkinsfile: Workaround failure with multistage Dockerfile

The Jenkins Docker workflow plugin fails badly when trying to inspect the built images when using a multi-stage Dockerfile.

That's because the plugins naively looks for the image name by taking all the contents after FROM , which means it breaks whether a stage name is introduced with AS stagename or even if more than a single space is used after FROM.

See https://issues.jenkins-ci.org/browse/JENKINS-44609

Invoking docker directly as suggested on the JIRA issue works around the problem.

Test run: https://jenkins.apertis.org/job/playground/job/em/job/apertis-lavaphabbridge-image/1/console