Remove unnecessary environment variables from dockerfile

[beforan]

I'm pretty sure slack integration doesn't require half the environment variables anymore, so we should determine which ones we can remove, and chuck them from the dockerfile.

[beforan]

The shell script I'm using atm to run the container sets the following environment variables:

• HUBOT_NAME
• HUBOT_SLACK_TOKEN
• HUBOT_SLACK_TEAM

But I think even some of those are unnecessary, as the integration nowadays specifies the name, and is added to a team. If I had to guess I'd say only the Slack token is needed.

[beforan]

confirmed only HUBOT_SLACK_TOKEN is required.