The dockerfile of this container has been copied from the official nginx repo (alpine-perl variant) and has been modified to add ModSecurity library (v3) + ModSecurity nginx connector.
You can refer to the official nginx image documentation for instructions on how to use this image.
When you provide your configuration you can enable modsecurity. Please refer to their wiki for documentation.
NOTE: no rules are shipped with this container, if you enable modsecurity you need to provide your own
If you're curious to know the difference from this dockerfile and the upstream one:
diff <(curl -fsL https://github.com/nginxinc/docker-nginx/raw/1.15.9/mainline/alpine-perl/Dockerfile) <(curl -fsL http://github.com/elisiano/docker-nginx-modsecurity/raw/master/Dockerfile)