[jib-cli-jar] Add option to specify creation time of container

[mpeddada1]

Required a bit of refactoring to use the Instants class which was initially implemented for the build command.

[chanseokoh]

Can we add a test that checks the default time of the built image (or something to that effect) when the option was not given?

[codecov]

Codecov Report

Merging #3050 (033bb33) into master (0643c90) will increase coverage by 0.01%.
The diff coverage is 80.00%.

@@             Coverage Diff              @@
##             master    #3050      +/-   ##
============================================
+ Coverage     71.07%   71.09%   +0.01%     
- Complexity     2305     2307       +2     
============================================
  Files           278      278              
  Lines          9751     9755       +4     
  Branches        989      989              
============================================
+ Hits           6931     6935       +4     
  Misses         2480     2480              
  Partials        340      340              

Impacted Files	Coverage Δ	Complexity Δ
.../java/com/google/cloud/tools/jib/cli/Instants.java	91.66% <0.00%> (ø)	1.00 <0.00> (?)
...e/cloud/tools/jib/cli/buildfile/BuildFileSpec.java	100.00% <ø> (ø)	22.00 <0.00> (ø)
...ud/tools/jib/cli/buildfile/FilePropertiesSpec.java	100.00% <ø> (ø)	9.00 <0.00> (ø)
.../main/java/com/google/cloud/tools/jib/cli/Jar.java	42.18% <100.00%> (+2.84%)	13.00 <2.00> (+2.00)
...a/com/google/cloud/tools/jib/cli/jar/JarFiles.java	95.45% <100.00%> (+0.21%)	3.00 <0.00> (ø)

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[chanseokoh]

Another option is to provide the default (e.g., 0?). That way, maybe creationTime can never be null?

Either way has pros and cons though.

[mpeddada1]

I did think about adding a default but I assumed this would be a little redundant since we already have a default value for this field in jib?

[chanseokoh]

Either is fine. At least we can document the default value in the option description.