Commit 8fed685f12 added a HEALTHCHECK
instruction in the Dockerfile. The healthcheck script calls http://localhost:8000/api/v3/status/, which fails if localhost is not in ALLOWED_HOSTS.
With this change, the healthcheck script is now a Django management
command. It reads Django's ALLOWED_HOSTS setting, grabs the first
element, and uses it in the "Host:" HTTP header when making a HTTP
request.
cc: #1051
