OOM due to healthcheck

[withinboredom]

This is related to #366

When containers are running at 99-100% of allocated memory, OOM killer comes along and kills frankenphp when the Dockerfile healthcheck runs:

HEALTHCHECK CMD curl -f http://localhost:2019/metrics || exit 1

The curl command only consumes a few MB of memory, but when frankenPHP already uses nearly all available memory, this is often enough to trigger the OOM killer.

We should either remove the healthcheck or provide instructions on how to disable the healthcheck in memory-constrained scenarios.

[withinboredom]

Fixed by #442