Updated Alpine Linux Pharo VM Docker image

Hi all,

I've updated the Docker image for pharo.cog.spur.minheadless VM
built on Alpine Linux. This version is built on Alpine Linux 3.12.

Some links:

- https://hub.docker.com/r/pierceng/pharovm-alpine
- https://github.com/pharo-contributions/Docker-Alpine/tree/master/vm.build
- https://github.com/PierceNg/opensmalltalk-vm/tree/pierce_alpine_839a5ca

This version removes the following plugins. I'm still thinking about some
of the others, especially the GUI-related ones. The idea is of course to
have the smallest possible set of plugins. Comments welcome.

- Security
- Drop
- Croquet
- DSAPrims
- JoystickTablet
- MIDI
- Serial
- StarSqueak
- InternetConfig

The output Docker image contains the Pharo VM only and is not runnable by
itself. It is intended to be used as a base to build your own Docker image
containing your application-specific Pharo image.

I'll be building a similar Docker image for Pharo's fork of the VM.

Pierce