Fix header issues identified in lesson review

[aturner-epcc]

Editorial review comments relating to headers from https://github.com/carpentries-lab/reviews/issues/15:

• The Introduction and Introducing the Docker Command Line ("Docker command line" section) episodes, and the Container Orchestration and Creating Containers on the Cloud extras use third-level headings for sections (heading lines beginning with ### ). Please adjust these to second level headings.

• The Using Docker with Github Actions extra episode contains several top-level headings (heading lines beginning with a single #) for sections within the page. Please adjust all of the heading levels in this extra episode down one level, # to ##, ## to ###, etc.

[aturner-epcc]

Closed as duplicate of #195