Dockerfile fails to build

[williballenthin]

There are some issues building the Dockerfile for radare2 today. They seem to stem from the node environment changing; however, I'm not super familiar with node.

First, npm is not getting installed. The shell script from https://www.npmjs.com/install.sh is run, but does not install correctly. This can be fixed by installing node v8.x, which packages npm with it.

Two, the r2pipe npm package depends on node-gyp, which has some issues installing in a docker container. We encounter the issue here: nodejs/node-gyp#454, and a fix is described here: nodejs/node-gyp#454 (comment). Ultimately, npm generally shouldn't be run as root, but I think in this case, its not a huge deal.