Fixed dockerfile to build ui

[CyrilDesch]

• NPM install and frontend build need to be present before the go build because the executor contain the frontend
• Clarify Dockerfile

[aviadhahami]

@CyrilDesch - heya and thx! :)
Are you sure about this? for when I tested it locally I had no problems when building like this...

[CyrilDesch]

I tried on Mac and Linux and I have this problem. Maybe your config on devcontainer solve the problem.

I base my opinion on the fact that there is no UI folder in the downloadable release :
https://github.com/nfx/slrp/releases/tag/v0.3.0

So it seems logical to me that it's inside the "build".

[aviadhahami]

its basically the same, im fine with it