add relative_url_root to webpacker config

[ipmsteven]

This PR will set WEBPACKER_RELATIVE_URL_ROOT as ActionController::Base.relative_url_root which partially solve this issue #1106

would you mind taking a look? @gauravtiwari
Thanks

[brunoOSI]

Please add this in, it is a bummer that you only can ship precompiled asset paths. It should allow some configuration!

[gauravtiwari]

Thanks @ipmsteven for this. Sorry for the delay.
I think this requires a bit more wiring to function correctly like appending relative root to publicPath. Will take a look at this soon.

[ipmsteven]

Hi @gauravtiwari

You are right, this PR only make relative_url_root available during webpack compile time. A bit more wiring need to be done to construct publicPath that respect relative_url_root.
I am thinking we could start with only exposing relative_url_root info and delegate the plumbing work to the user of webpacker for now.

so with that being said above, If you agree with the proposal of adding WEBPACKER_RELATIVE_URL_ROOT to webpacker compile environment,

we could either
Plan 1:

1. merge this PR that only ships WEBPACKER_RELATIVE_URL_ROOT first
2. have separate PR to do the wiring work as you mentioned above

OR

Plan 2: Have one PR handle both 1 and 2

I am fine with either one, what do you think? 😃

[gauravtiwari]

Thanks and sorry it took a while to get this one merged. I will take a look at part 2.

[ipmsteven]

Thanks! @gauravtiwari