Dockerfile improvements: size, verbosity and style

[pataquets]

• Size: add --no-cache switch. Not useful on build, Docker's build caching is responsible of caching.
• Add some -v switches here and there to add verbosity and provide helpful info when troubleshooting.
• Move export statement to an ARG Docker clause, valid for all the build (only in its own FROM scope).

[codecov[bot]]

Codecov Report

Merging #360 (09793b9) into main (62e958f) will decrease coverage by 1.12%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #360      +/-   ##
==========================================
- Coverage   89.21%   88.09%   -1.13%     
==========================================
  Files          25       25              
  Lines        1957     1957              
==========================================
- Hits         1746     1724      -22     
- Misses        211      233      +22     

Impacted Files	Coverage Δ
src/gphotos_sync/Utils.py	89.65% <0.00%> (-5.18%)	:arrow_down:
src/gphotos_sync/Main.py	83.58% <0.00%> (-5.13%)	:arrow_down:
src/gphotos_sync/GooglePhotosDownload.py	79.89% <0.00%> (-2.02%)	:arrow_down:
src/gphotos_sync/Checks.py	98.44% <0.00%> (-1.56%)	:arrow_down:
src/gphotos_sync/GooglePhotosMedia.py	83.33% <0.00%> (-1.39%)	:arrow_down:
src/gphotos_sync/LocalData.py	88.73% <0.00%> (-0.94%)	:arrow_down:

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[gilesknap]

Merging - thanks for contribution.