(docs): add more detailed godoc

[coveralls]

Pull Request Test Coverage Report for Build 646539411

• 18 of 22 (81.82%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 74.872%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
image.go	2	4	50.0%
volume.go	4	6	66.67%
<!--	Total:	18	22	81.82%	-->

Files with Coverage Reduction	New Missed Lines	%
container.go	1	75.31%
<!--	Total:	1		-->

Totals
Change from base Build 643441550:	0.3%
Covered Lines:	146
Relevant Lines:	195

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💛 - Coveralls