CI: Increase memory limit of docker container for gitlab runner (for redox)
Job #27838 failed for 18922255:
redoxer: copying '.' to '/root'
qemu-system-x86_64: cannot set up guest memory 'pc.ram': Cannot allocate memory
## redoxer (failure, qemu exit status ExitStatus(ExitStatus(256)) ##
The memory limit of the docker runner appears to be too low.