Add missing docker_args

[soderluk]

• Add missing parameter docker_args to release function
• Fix docker_args help messages to include type

[soderluk]

We discussed with @joakimnordling about the plural form of docker_args and came to the conclusion after inspecting the actual CLI args: --docker-args foo=bar --docker-args bar=baz vs --docker-arg foo=bar --docker-arg bar=baz, that the singular form is clearer, and it complies with e.g. the component argument form.

[soderluk]

@joakimnordling: Readme updated

[lietu]

There's some inconsistency between --docker-arg=foo=bar and --docker-arg foo=bar in different bits of documentation - maybe better to use each form once on every example invocation or some such?