automated merge to deploy branch sometimes fails

[rhelmer]

Currently we have Circle CI perform the following tasks:

1. build/test, if successful then
2. commit built artifacts to a deploy branch, first merging any changes in the origin repo
3. push deploy branch to the origin repo

Mozilla SRE has a CI job that watches our deploy branch, and then runs firebase deploy.

This works most of the time, however occasionally step 2 fails, for example: https://app.circleci.com/pipelines/github/mozilla-rally/rally-web-platform/571/workflows/bc01c77b-4e67-4a1c-b846-8574e2fa9f98/jobs/815

#!/bin/bash -eo pipefail
git fetch origin && git merge -s ours deploy && git push origin deploy

Warning: Permanently added the RSA host key for IP address '140.82.114.3' to the list of known hosts.
remote: Enumerating objects: 10, done.        
remote: Counting objects: 100% (10/10), done.        
remote: Compressing objects: 100% (7/7), done.        
remote: Total 10 (delta 3), reused 6 (delta 3), pack-reused 0        
Unpacking objects: 100% (10/10), 263.39 KiB | 9.08 MiB/s, done.
From github.com:mozilla-rally/rally-web-platform
   ef90f56..28273e3  deploy     -> origin/deploy
 + 9de1259...90405c5 dependabot/npm_and_yarn/minimist-1.2.6 -> origin/dependabot/npm_and_yarn/minimist-1.2.6  (forced update)
Already up to date.
To github.com:mozilla-rally/rally-web-platform.git
 ! [rejected]        deploy -> deploy (non-fast-forward)
error: failed to push some refs to 'git@github.com:mozilla-rally/rally-web-platform.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

Exited with code exit status 1

CircleCI received exit code 1

We should understand why this is happening and fix it in the short-term, long-term we should figure out if this is the deployment strategy we want to continue with.