task runtime skeleton

[mlin]

Initial skeleton of the local task runtime, with proofs of concept for docker container management, file path mapping, and I/O stdlib functions like stdout() and write_lines().

[coveralls]

Pull Request Test Coverage Report for Build 616

• 271 of 294 (92.18%) changed or added relevant lines in 6 files are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.4%) to 96.121%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
WDL/Value.py	17	23	73.91%
WDL/runtime/task.py	232	249	93.17%

Files with Coverage Reduction	New Missed Lines	%
WDL/Tree.py	6	98.24%

Totals
Change from base Build 595:	-0.4%
Covered Lines:	3172
Relevant Lines:	3300

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💛 - Coveralls