Dockerfile bug fixes
Dockerfile bug fixes

npm install wasn't actually running - octohost/nodejs was throwing
an error on npm install because it couldn't drop privs in the
workdir. Adding --unsafe-perm seems okay in a docker context.

If there is no SLACK_CHANNELS env var, slackin was failing to parse
the command line. I updated it to match the Heroku command line.

IOT-DSA/slackin,mike1pol/slackin,Code-T/slackin,thanosIrodotou/slackin,Milwaukee-Slack/Milwaukee-Slack-Invite-Website,PHPSP/slackin,18F/slackin,mbbroberg/slackin,ouluwebdev/slackin,Venpath/slackin,lemonlabsuk/slack-invite-page,Venpath/slackin,PHPSP/slackin,IOT-DSA/slackin,rauchg/slackin,emccode/community,emccode/community,ouluwebdev/slackin,atom/slackin,mike1pol/slackin,devopsnz/slackin,Ineeza/luka_ineeza_slakin,martindale/slackin,JLPcommunity/join-slackin,szkl/slackin,avaclancheuk/slackin,emccode/community,szkl/slackin,18F/slackin,TechnologyMasters/technologymasters.herokuapp.com,Milwaukee-Slack/Milwaukee-Slack-Invite-Website,TibiaPF/slackin,JoomSavvy/aauj,edorgeville/slackin-fr,Code-T/slackin,lemonlabsuk/slack-invite-page,avaclancheuk/slackin,TibiaPF/slackin,TechnologyMasters/technologymasters.herokuapp.com,mbbroberg/slackin,devopsnz/slackin,thanosIrodotou/slackin,edorgeville/slackin-fr,rauchg/slackin,thanosIrodotou/slackin,Ineeza/luka_ineeza_slakin