build: Specify Dockerfile version

[alexec]

Needed for the mount option on some build system (e.g. ours).

[codecov]

Codecov Report

Merging #5689 (8aeeffd) into master (e3d1d1e) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5689      +/-   ##
==========================================
+ Coverage   46.86%   46.89%   +0.03%     
==========================================
  Files         244      244              
  Lines       15209    15209              
==========================================
+ Hits         7127     7132       +5     
+ Misses       7172     7171       -1     
+ Partials      910      906       -4     

Impacted Files	Coverage Δ
cmd/argo/commands/get.go	56.31% <0.00%> (-0.65%)	⬇️
workflow/controller/operator.go	71.11% <0.00%> (+0.26%)	⬆️
cmd/argoexec/commands/emissary.go	50.00% <0.00%> (+1.56%)	⬆️

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