Build and use bot Docker image directly in firefox-ci

[La0]

CopyCat of mozilla/code-review#475

Instead of pushing the bot's docker image on dockerhub, we need to build it with DinD on the firefox-ci instance, mark that task with a route, and use the testing/production images in the hook.
This will allow us to ship releases without any intervention from releng (see Bug 1594102)

The plan is :

• Enable Firefox-ci Github app on the repo - (integration requested)
• Get necessary scopes to build & index the image on firefox-ci Bug 1617635
• Update .taskcluster.yml to build DinD image on firefox-ci instance
• Add route to the DinD build task on firefox-ci instance
• Update hooks to use the docker images with index-image

[La0]

I merged this hotfix to silent the firefoxci taskcluster trigger as we do not have the scopes yet.

[La0]

The bot image is now available under https://firefox-ci-tc.services.mozilla.com/tasks/index/code-analysis.v2.code-coverage.branch/master

[La0]

All set 🎉