Enable Mattermost notifications for che release stages

[nickboldt]

Is your enhancement related to a problem? Please describe.

Communication of release status is sketchy and manual. We could automate it so that it's more obvious what's happening.

Describe the solution you'd like

For every successful completion of a PHASE of the che-release process, send a mattermost notification w/ link to relevant status content:

eg.,

che-machine-exec 7.26.0 released :: https://quay.io/eclipse/che-machine-exec:7.26.0
This release: https://github.com/eclipse/che-machine-exec/actions?query=workflow%3A%22Release%22
Overall release: https://github.com/eclipse/che-release/actions?query=workflow%3A%22Release%22

GH action built-in: https://github.com/marketplace/actions/mattermost
MM channel to notify: https://mattermost.eclipse.org/eclipse/channels/eclipse-che-releases

Channel would therefore get about 10 announcements, once a week, for each release.

[nickboldt]

Initial prototype for announcing the overall release is done:

• https://github.com/eclipse/che-release/actions/workflows/release-send-mattermost-announcement.yml
• https://github.com/eclipse/che-release/blob/master/.github/workflows/release-send-mattermost-announcement.yml

With this, we can:

• announce when the whole release is done (manual workflow trigger, to remind people about issue triage) : DONE

In future, we could:

• announce individual stages in the release are done (API call when stuff is done in other repos)
• announce when everything is done (except the PRs that need merging) (when operator and chectl are merged)

[nickboldt]

For example... eclipse-che/che-machine-exec#134 (requires uploading the MATTERMOST_WEBHOOK_URL secret to work)

[nickboldt]

PRs:

• machine-exec: https://github.com/eclipse/che/issues/18906 notify mattermost when release done che-machine-exec#134 (merged)
• devfile reg: add MM message che-devfile-registry#342
• theia: add MM message che-theia#1025
• plugin reg: add MM message che-plugin-registry#878
• dashboard: add MM message che-dashboard#186
• server: add MM message #19272
• operator: add MM message che-operator#712
• chectl: feat: add MM message che-incubator/chectl#1129

[nickboldt]

che-theia notification failed because:

2021-03-11T12:07:03.2004173Z ##[command]/usr/bin/docker build -t 5588e4:973db1779c234cc89fcd708ca4c62af8 -f "/home/runner/work/_actions/mattermost/action-mattermost-notify/master/Dockerfile" "/home/runner/work/_actions/mattermost/action-mattermost-notify/master"
...
2021-03-11T12:08:39.5760632Z Successfully tagged 5588e4:973db1779c234cc89fcd708ca4c62af8

but then

docker image prune -a -f
...
2021-03-11T12:09:25.1531776Z untagged: 5588e4:973db1779c234cc89fcd708ca4c62af8

-- https://pipelines.actions.githubusercontent.com/r9pXOaZgznilET3xqjzkK4PuFIyiUj24cGg6t9ne59Rxdg3CN0/_apis/pipelines/1/runs/3327/signedlogcontent/3?urlExpires=2021-03-11T14%3A57%3A47.8776142Z&urlSigningMethod=HMACV1&urlSignature=XYWivSWtridUwtQgO9vGl%2Fq0F3AZ1MhHk4ZJGP6AwDQ%3D

So the container created at the start of the run is untagged before it can be used at the end.

Workaround:

• move MM notification to a separate job step so it's in its own container che-theia#1029