Add explicit NPM, ruby-bigdecmial and ruby-etc dependency

It seems that recently the Node package on Alpine has removed the NPM CLI, so now we need it explicitly.

It also seems that via 6563 we need the bigdecimal package explicitly added, which in Alpine's case can be done via the ruby-bigdecimal package.

It also seems like we now also need the etc gem explicitly pinned, whereas before it's been included in the rubygems package.