Publish: better support for app settings for different environments

[sayedihashimi]

From dotnet/AspNetCore.Docs#2029 (comment) @benzhi2011 has requested that we have better support for app settings for different environments. In the past he used web.config transforms and was pretty happy with that.

For ASP.NET Core we should have a better story than what we have today. We will need to think about this, and create a proposal here. Right now I'm busy with some other time critical work so I can't do that now, but I'll get to it over the next few weeks.

[benzhi2011]

web.config transforms is a very powerful thing, I think it will be great .net core can continue to support this.

[guardrex]

Related: There is a nice community bit from @nil4 at https://github.com/nil4/dotnet-transform-xdt

[nil4]

Also related: https://github.com/OctopusDeploy/Octopus.Web.Xdt by @matt-richardson (see https://github.com/dotnet/corefx/issues/11196 for context).

[sayedihashimi]

We are planning to port xdt to core. This issue is more the idea of "config transform". Since we don't use xml files for config xdt doesn't directly apply.

[ghost]

A convenient way to handle multiple environment should be to add a rule to publishOptions to ignore files on different environment.