Switch to node:slim for Dockerfile base image

[vdice]

This change was motivated by the following error encountered when running the image built using the pre-existing octohost/nodejs-based Dockerfile:

$ docker run --rm vdice/slackin:octohost-base sh -c './bin/slackin --version'

/srv/www/node_modules/args/dist/index.js:39
class Args {
^^^^^
SyntaxError: Unexpected reserved word
    at Module._compile (module.js:439:25)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Object.<anonymous> (/srv/www/bin/slackin:4:12)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)

Whereas the image built off of the official node:slim variant (change in this PR) works great:

$ docker run --rm vdice/slackin:node-slim-base sh -c './bin/slackin --version'
0.13.0

A nice reduction of final image size is achieved as a bonus:

$ docker images | grep slackin
vdice/slackin                     node-slim-base      daef1e94ba1a        5 minutes ago       278.7 MB
vdice/slackin                     octohost-base       c74c39eadc07        7 minutes ago       589.3 MB