fix: ensure call to GCP only happens when cloud build is enabled

[jsilva]

Describe your changes

As we need to be able to build and push containers in an environment without access to GCP Artifact Registry/Cloud Build, the call to get project_number in docker service can only happen when we already know cloud build and GCP access env is allowed.

This PR moves the call to the single function that uses it, instead of the init

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 74.46%. Comparing base (2dc036b) to head (2244019).

Files	Patch %	Lines
src/wanna/core/services/docker.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
- Coverage   74.50%   74.46%   -0.04%     
==========================================
  Files          58       58              
  Lines        2867     2867              
==========================================
- Hits         2136     2135       -1     
- Misses        731      732       +1     

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[racinmat]

LGTM