Update to use latest available nginx images (#30)

* Update to use latest available nginx images

While here:
 - simplified Alpine installation by using nginx prebuilt image;
 - added Makefile for convenience;
 - added podman support.

* fix: use short signal names in entrypoint.sh

This allows to use the same script under sh, bash, dash,
and probably other shells as well.

* fix: exit on agent configuration check failure

This brings the previous behavior, avoiding container to keep running
without agent when failures occur at initialization stage.

While here, removed SIGHUP from a list of signals for stopping services.

* fix: run amplify-agent under nginx user

This change is essential for both rootless podman and usual docker
environments, as agent which is running under root gets EPERM errors
from resource.prlimit() while trying to collect various metrics
from nginx worker processes running under nginx user.

At the same time, it makes impossible to use /dev/stdout to output
agent's log to (good explanation is available at [1]), thus entrypoint
script now launches "tail -F" instead.

[1] moby/moby#31243

Author: defanator