Docker executer: allow to configure `--shm-size`

Description

Making shm-size configurable can help to run docker images with high pressure on shared memory (like selenium images for automated browser testing).

It is configured by the "--shm-size" parameter and defaults to 64m.

Edit: This should also change the shm size of all services, not only the main image.

Proposal

Allow to set "--shm-size" by an entry in [runners.docker] section, e.g. shm_size = 1g. Now the main image and all services use the --shm-size parameter.