Feature Request: Make "--shm-size" configurable

[ngraf]

Given I am in an Linux elastic GoCD agent
When I run more than 3 acceptance tests in parallel with headless chrome
Then the tests crash because I ran out of space in folder /dev/shm (64 mb = docker's default value).

(See issue elgalu/docker-selenium#20)

Acceptance criteria

• The plugin enables the user to configure the size of /dev/shm

Info

• You can change size of /dev/shm by f.e. running docker run --shm-size=512mb
• See official documentation for --shm-size here: https://docs.docker.com/engine/reference/run/#runtime-constraints-on-resources)