This reverts commit d85acc46.

This reverts merge request !284

Fixes #397
Fixes #423

* Rename StartPlugin -> PluginRun
* Rename PluginRun.container -> PluginRun.containerImage
  This terminology is technically better as docker runs
  container _images_ in _containers_.
* Reset database (via name change) due to breaking
  change in migration files content.
  (This could be avoided, but no need for now.)
* Update v3 -> v4 because a rename is a breaking change

Nobody used this API at this moment
because API wasn't working until 2 commits ago.

While not clean to change without deprecation warnings and all,
it should be safe to do so now.

This will differentiate the address on which Pod listens to requests,
and the Pod address that services will see in order to make the callback.

This commit only formalizes the difference and makes it clearer,
it was partly already the case before.

Additionally, default port value of 3030 was moved to a constant,
and docs were improved.