fix(instance): remove volume name when using image

[Codelax]

Fix #1384

[codecov-commenter]

Codecov Report

Merging #1390 (5bc5903) into master (28b5b4b) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1390      +/-   ##
==========================================
+ Coverage   66.32%   66.34%   +0.01%     
==========================================
  Files         124      124              
  Lines       19717    19727      +10     
==========================================
+ Hits        13078    13088      +10     
  Misses       5495     5495              
  Partials     1144     1144              

Impacted Files	Coverage Δ
scaleway/resource_instance_server.go	73.38% <100.00%> (+0.32%)	⬆️

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