Allow to override build date with SOURCE_DATE_EPOCH

[bmwiedemann]

Allow to override build date with SOURCE_DATE_EPOCH
in order to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

This date call only works with GNU date (probably not a problem since kubernes is pretty Linux-specific).

Also use UTC to be independent of timezone.

[coveralls]

Pull Request Test Coverage Report for Build 161437726

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.41%

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Totals
Change from base Build 159317291:	0.0%
Covered Lines:	1074
Relevant Lines:	1504

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💛 - Coveralls

[dougbtv]

Thanks for the contribution and the links to describe your intention, appreciate it!