ISSUE-156 Add service to notify Slack of Docker events

[lewismc]

This issue was really nice to work on :) I simply

• created a new Slack channel in Tagbase Upgrades Slack workspace
• setup an Incoming WebHook on the tagbase upgrades Slack workspace and obtained the WebHook URL.
• created a webhook entry in .env and added the Webhook URL
• build and then up to create a docker composition... this immediately sent messages

[lewismc]

I've also documented this at https://github.com/tagbase/tagbase-server/wiki/Operations#real-time-docker-event-notifications