Evaluate Docker's availability on target platforms

Primary target build platforms are: Debian stable (+ backports), Debian testing/sid, latest Ubuntu LTS, latest Ubuntu.

Blueprint: https://tails.boum.org/blueprint/evaluate_Docker/

Parent Task: #7530 (closed)

Related issues

• Blocks #7533 (closed)

Original created by @intrigeri on 7532 (Redmine)