Docker -> Container

[doshitan]

There's nothing Docker-the-product specific to the usage necessarily, though
some of the file handling by a root daemon like Docker helps. Generalize the
naming of things in the project and parameterize which container command is used
by the Makefile.

Process checklists

Pre-review request

• I have provided a clear title and description of the changes, with
  motivation and rationale for why things need change, notes on obvious
  alternatives considered, and links to related work. Formatted according to
  project guidelines.
• I have personally reviewed every line change.
• I have provided evidence the changes function correctly.
• I possess the copyright to the changes such that they can be licensed
  under the project's terms.

Pre-merge

• I have ensured the title and description reflect the final state of
  changes.
• I have tested and confirmed final changes function correctly.

Testing

Building

[...]

Running

(with set -x)