CI: Add build instructions for docker

Signed-off-by: Alexander Diewald <diewald@fortiss.org>
3 jobs for master in 14 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #49569
build

00:10:07

 
  Test
passed #49570
test

00:03:22

 
  Deploy
passed #49571
deploy

00:01:05