build: Add mono and berp to docker (#1001)

To make cucumber/common#771 possible we need both mono and berp. Currently mono isn't installed and berp is checked in as a binary executable. Both are less then ideal. 

Currently mono isn't supported on the same version of alpine as .NET Core SDK 2.2.2. To avoid messing around too much with APK which doesn't support mono yet I've changed the base image to `ubuntu:20.04`. This makes it easier to install mono.

Additionally we can now also use the node version manager to manage the node and npm versions. They'll be installed without the need for global permissions. This solves another long standing build headace.

The make file will now use `mono  /var/lib/berp/1.1.1/tools/net471/Berp.exe -g gherkin.berp` which is less out of the box then it was before, but the exact installation instructions for berp and mono can be found in the `Dockerfile`.

go/.gitignore

@@ -13,4 +13,5 @@ dist/
 dist_compressed/
 *.bin
 *.iml
-core.*.!usr!bin!upx
+# upx dist/cucumber-gherkin-openbsd-386 fails with a core dump
+core.*.!usr!bin!upx-ucl