Add support for web target

[Bluebugs]

Is your feature request related to a problem? Please describe:

Current develop branch of Fyne does provide a new target for packaging: web which does use go 1.17 to compile a wasm binary and gopherjs for generating a JS output (for use by Chrome browser).

Gopherjs is only supported on Linux. This is where fyne-cross would help by providing all other platform with a working solution.

Is it possible to construct a solution with the existing API?

Hopefully yes.

Describe the solution you'd like to see:

Add gopherjs in the docker image and add a new target.