fix(instance): set image and root_volume.volume_id to exactly one of

[Codelax]

Many snippets are coming in a next documentation PR

[codecov-commenter]

Codecov Report

Merging #1598 (b4a7d73) into master (53ac552) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1598      +/-   ##
==========================================
+ Coverage   67.80%   67.82%   +0.01%     
==========================================
  Files         139      139              
  Lines       22686    22687       +1     
==========================================
+ Hits        15383    15387       +4     
+ Misses       5978     5976       -2     
+ Partials     1325     1324       -1     

Impacted Files	Coverage Δ
scaleway/resource_instance_server.go	73.34% <100.00%> (+0.03%)	⬆️
scaleway/retryable_transport.go	77.50% <0.00%> (+7.50%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.