hg.nginx.org is unavailable, breaking Dockerfile

[jvanheesch]

Is your feature request related to a problem? Please describe

Our build requires an nginx container with lua. Based on these excellent instructions, we use this Dockerfile. Right now, our build is failing because https://hg.nginx.org/xslscript/raw-file/01dc9ba12e1b/xslscript.pl is down.

Describe the solution you'd like

To reduce the chance of build failures, perhaps a highly available read-only github mirror could be considered (similar to https://github.com/nginx/njs).