Varnish is an HTTP accelerator. It is an open source reverse proxy server for HTTP protocol which stores cached pages in virtual memory.
Main configurations are available as variables (you can see it with options
command).
You can add your VCL file by VARNISH_VCL_CONF
variable:
docker run --name varnish \
-v /path/to/default.vcl:/etc/varnish/default.vcl:ro \
-e VARNISH_VCL_CONF=/etc/varnish/custom.vcl \
saidsef/docker-varnish