Allow location of nginx.conf to be configurable by environment variable

[robdimsdale]

Currently the location of nginx.conf is hard-coded. This buildpack would be more resilient and flexible if this were configurable.

There are a couple of use-cases for this:

• Upstream buildpacks (e.g. php-nginx) can instruct this buildpack as to where it can find the nginx config file that was created earlier in the build process
• Application developers can instruct this buildpack as to where their nginx configuration file is, if it is not in the root of their application directory.

I propose the following configuration:

• This buildpack optionally looks for the environment variable called BP_NGINX_CONF_LOCATION
• If the environment variable is unset, fall back to the default location of /workspace/nginx.conf
• If the environment variable is set to a relative filepath, assume that it is relative to /workspace/
• If the environment variable is set to an absolute filepath, use it as-is (i.e. relative to /

I am happy to move this to an RFC if that process is preferred.

[sophiewigmore]

We're looking for any input that the @paketo-buildpacks/web-servers-maintainers may have on this

[arjun024]

Proposal Sounds good to me.

• There was a related discussion about using bindings to provide nginx.conf (Including Service Bindings for nginx.conf and mime.types #281 (comment)) - not sure if you have seen this or if it will provide any extra input but worth checking out if you haven't.

• Also, I have heard community interest about getting nginx buildpack to work on the tiny stack (e.g.). It would be nice to think if we have a desire for future tiny support whether using more environment variables would be tricky without a shell (direct process with configure binary running as exec.d). It's probably fine but worth checking out.

None of these are blockers. Just wanted to do a quick brain dump in case if it's helpful

[TisVictress]

@arjun024 There is currently a PR (#343) that makes the configure binary run as exec.d. I don't think using environment variables would create an issue because we should be able to use them within the configure binary at launch.

@robdimsdale I like the name you proposed, BP_NGINX_CONF_LOCATION, and it make sense to have a default value.