It seems that the issue with not being able to find webpack-cli is
due to some behavior with an older version of npm. My local devel-
opment environment has npm 7.6.x and so does the Node.js 15 conta-
iner, while the Node.js 12 and 14 containers have npm 6.14.x.
Updating npm fixes the issue with `npx webpack` not being able to
find webpack-cli's webpack command (perhaps it installs the peer
dependency automatically, I don't know).
Node.js v10 will cease to be LTS next month and I haven't even used
it for a while already, so let's change this to only run on 12, 14,
and 15, which are the current active and LTS releases.
See: https://nodejs.org/en/about/releases/