Allowing to override the image used in docker-compose files can allow
for richer and more easy local development/debugging sessions. The
docker image building process is already using the latest tag anyway,
making it really easy to build an image with a (set of) specific
commits. Using the above built image with the provided docker-compose
files isn't doable without modifications though. Add that support via
environmental variables.
'start_period' for 'healthchecks' was added in Compose file version 3.4,
https://docs.docker.com/compose/compose-file/compose-versioning/#version-34
Bump docker-compose files' versions from 3.3 to 3.4
Without this, docker-compose (1.25 at least) returns
ERROR: The Compose file './contrib/docker-compose/caddy.yml' is invalid because:
services.db.healthcheck value Additional properties are not allowed ('start_period' was unexpected)
