Enable Alpine official RID

[janvorli]

This is a leftover from the Alpine enabling changes that I've forgotten
to make after corefx and coreclr official builds were enabled.

[janvorli]

cc: @MichaelSimons

[janvorli]

@eerhardt I have removed this from my original change that enabled Alpine support in core-setup based on your feedback, but forgotten to add it after the official builds of coreclr / corefx were out.

[eerhardt]

Looks good. I verified we have corefx and coreclr builds for alpine.3.6 for the current versions we are on. So this should work.

[eerhardt]

@janvorli - are we enabling Jenkins legs for alpine and rhel6?

[janvorli]

@eerhardt I've enabled RHEL 6 in corefx only so far (optionally). I've discussed it with @mmitche some time ago and the recommendation was to wait with coreclr / core-setup until the ci moves to the new way of using pipelines for CI build.