Upgrade to debian bookworm (ruby 3.1)

[oppegard]

What is this change about?

This upgrades the Dockerfile to Debian bookworm.

Please provide contextual information.

The primary reason for the change is to address #191. We need a newer version of ruby that supports the latest release of the bosh-template gem used in RSpec suites.

Please check all that apply for this PR:

• [ ] introduces a new task
• [ ] changes an existing task
• [x] changes the Dockerfile
• [ ] introduces a breaking change (other users will need to make manual changes when this is released)

Did you update the README as appropriate for this change?

• [ ] YES
• [x] N/A

How should this change be described in release notes?

Upgrade to Debian bookworm (ruby 2.7 -> 3.1) in Dockerfile.

What is the level of urgency for publishing this change?

• [ ] Urgent - unblocks current or future work
• [x] Slightly Less than Urgent

Tag your pair, your PM, and/or team!

cc @ctlong