fix(cli): do not pass the force-build flag to docker

[Panaetius]

Fixes #3612

[coveralls]

Pull Request Test Coverage Report for Build 6810240853

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• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.01%) to 82.838%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
renku/core/session/docker.py	1	2	50.0%

Files with Coverage Reduction	New Missed Lines	%
renku/core/workflow/activity.py	1	93.61%
renku/ui/cli/init.py	1	96.9%
renku/ui/cli/service.py	1	28.57%

Totals
Change from base Build 6800487347:	-0.01%
Covered Lines:	25452
Relevant Lines:	30725

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💛 - Coveralls