MLPAB-2806 - pull spec during docker build

[andrewpearce-digital]

Purpose

Drop requests to raw.githubusercontent.com at the network firewall

Fixes MLPAB-2806

Approach

• Download spec file for imposter during docker build
• use dev docker compose override to mount volume for local dev of mock-pay

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.05%. Comparing base (f0436ef) to head (6e3693c).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1729      +/-   ##
==========================================
+ Coverage   95.03%   95.05%   +0.01%     
==========================================
  Files         285      285              
  Lines       16487    16487              
==========================================
+ Hits        15669    15672       +3     
+ Misses        646      644       -2     
+ Partials      172      171       -1     

Flag	Coverage Δ
unittests	95.05% <ø> (+0.01%)	⬆️

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