This requires us to duplicate environment variables.

The commit also disables --reproducible mode for kaniko – our images are
not reproducible in the first place (because we run apt-update) and
using the option leads to weird dates being shown for the images.

Closes #58

This is required in order to have -dev packages available during our
build stage. Apparently, this should have never worked and the upstream
image has just been cleaned up to align with documentation.

See https://github.com/docker-library/ruby/pull/497

Closes !25

This includes a two-part update of solid_queue via 0.6.0 as requested in the changelog.

This allows for safer zero-downtime upgrades.