feat(docker): Docker Feature Flag

[refcell]

Description

Creating this issue as a placeholder - it may be useful/necessary to put all docker-dependent code behind a docker feature flag.

Thinking: the op-composer internally has a Docker bollard reference that we may not want to "leak" to the stages crate unless the docker flag is enabled (could be by default unless necessary).

Effectively, a good start would be to add a docker feature flag to the stages crate and hide all docker-dependent code in the crate behind this flag.