Just for reference, the health check configuration that can be used in a docker-compose.yaml file is:

    healthcheck:
      test: ["CMD", "wget", "http://localhost:8008/_synapse/admin/v1/server_version", "--output-document=/dev/null"]

Docker runs this within the container, so wget (or curl, or busybox, or a custom client) needs to be available for it to work.

@schildbach out of interest, what purpose does this healthcheck serve that is not simply covered by ensuring that the container is running? I guess it would detect if synapse had somehow locked up, but is that a common problem? It feels to me like the healthcheck just adds another failure mode.

It makes sure Synapse has started up and is (still) responding to requests.

Ideally, the app itself would implement whatever it thinks is reasonable to check. For example, for Postgres there is the pg_isready command. Such a client is what I meant with "custom client".

Curl was added to docker in #8147, so this is probably superfluous now.