Fix 510: tweak Dockerfile to work around upstream environment issue with Ruby 2.7.0

[rrazor]

Overview

This PR resolves #510 by working around the non-apk-provided ruby 2.7.0 from alpine:ruby using environment variables.

I did the following:

• Redefined GEM_HOME from upstream to be accurate
• Removed gem install bundler as it is provided upstream in /usr/local/bin
• Added build-base to apk del to be symmetric with the temporary apk add

Testing

There are no automated tests but I have manually verified that the following results in JSON output:

$ docker build -t ssh_scan .
$ docker run -it ssh_scan /app/bin/ssh_scan -t sshscan.rubidus.com
[...]

Additional steps

A better way to fix this may be to submit an upstream PR to the maintainers of alpine:ruby so that they can set the correct GEM_* environment variables according to the source-built version of ruby they choose to use.

For now, however, I believe this will correct the issue as reported.

[claudijd]

Thanks for the fix!