Rebuild docker image to get npm>5.5.1 to allow self updating

[olizilla]

I tried to update the Makefile in peer-pad to use exactly [email protected] for it's install and build phase as npm > 5.7 now has a specific ci command designed for fast, reproducible installs on CI, where it doesn't have to worry about merging the deps tree with an existing node_modules dir. (blog)

Specifically, I wanted to

build:
  npm install -g [email protected]
  npm ci
  npm run build

That fails right now because the ci-websites image includes a version of node 8 that was bundled with npm 5.5.1 which has a bug that prevents it from being able to self update npm. More recent versions of node 8 are bundled with npm 5.6 or higher that doesn't have that issue. More info here npm/npm#16807 (comment)

Of note, this Dockerfile doesn't pin the version of node or npm, so it's just a case of building an publishing the image, no code change is required.

In related news does any one have an objection to a PR that pins the image to a specific version of node and npm? It would help when tracking down these sorts of weird, build related issues: peer-base/peer-pad#150