Improve error handling of outgoing webhook notifier (#7)

Signed-off-by: Chris O'Brien <[email protected]>

Extract Slack webhook values to config (#6)

* Fix Dockerfile build paths

Signed-off-by: Chris O'Brien <[email protected]>

* Move Slack webhook config to env vars

Signed-off-by: Chris O'Brien <[email protected]>

* Add new env vars to README

Signed-off-by: Chris O'Brien <[email protected]>

Readme updates

Signed-off-by: Jim Weber <[email protected]>