The Docker build image for `equuleus` no longer builds since the release of `dotenv` 3.0, which requires `ruby` >= 3.0, not available in the Debian `buster` repo. We should pin the `fpm` gem install version to 2.8.1.
```
#65 [60/66] RUN gem install --no-document fpm
#65 15.04 ERROR: Error installing fpm:
#65 15.04 The last version of dotenv (>= 0) to support your Ruby & RubyGems was 2.8.1. Try installing it with `gem install dotenv -v 2.8.1` and then running the current command again
#65 15.04 dotenv requires Ruby version >= 3.0. The current ruby version is 2.5.0.
#65 15.05 Successfully installed rexml-3.2.6
#65 15.05 Successfully installed stud-0.0.23
#65 ERROR: process "/bin/sh -c gem install --no-document fpm" did not complete successfully: exit code: 1
```