Out-of-order CI builds

[farski]

It's possible for two builds of the default (main) branch of a repo to be running at the same time, and have the older build (i.e., older code) finish second, which means the newer code artifact (e.g., Docker image) would be replaced by an artifact resulting from older code. In other words, to CD, the older code would look newer, and get deployed instead.

[farski]

I added a note to the readme about this.

The solution to this would be something like:
Right before post_build.sh publishes something to ECR or S3, get a list of the recent SHAs from GitHub, and look up tags/objects for each. Find the ones that exist, and then look to see if a tag/object exists for a commit that is newer than the SHA of the current build exists. If so, pushing the new image/object could be skipped.