The byjg/easy-haproxy image polls Docker for label changes every 10
seconds by default, which races with the haproxy bats suite and
intermittently produces curl exit 7. Expose `refresh-conf` as a
global-only haproxy property that maps to `EASYHAPROXY_REFRESH_CONF`,
lower it to 2 seconds in the bats setup, and wrap the localhost HTTP
assertions in a retry loop so checks wait for haproxy to converge
rather than failing on the first attempt.
Dokku uses a few external images for various bits of functionality, but these must be manually updated every so often due to being referenced in code.
By moving the references to Dockerfiles, we can take advantage of dependabot to issue automatic PRs when they are out of date.
