fix(core): update template checksum when updating Dockerfile

[m-alisafaee]

Description

We check Dockerfile changes in each commit and if they are only Renku version updates, we mark the Dockerfile as not being modified by users. Before doing this, we first check its checksum and checksum at time that the template was set to see if they match with the stored checksum; if so, we skip the git history traversal.

This PR also refactors templates_source fixture to use actual business models instead of a dummy template source.

Fixes #3346

[coveralls]

Pull Request Test Coverage Report for Build 4376329881

• 220 of 264 (83.33%) changed or added relevant lines in 27 files are covered.
• 17 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+0.2%) to 86.686%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
renku/core/git.py	0	1	0.0%
renku/core/migration/m_0010__metadata_fixes.py	0	1	0.0%
renku/core/session/session.py	0	1	0.0%
renku/core/migration/migrate.py	15	17	88.24%
renku/command/schema/activity.py	11	14	78.57%
renku/command/schema/project.py	8	11	72.73%
renku/core/migration/models/v9.py	8	11	72.73%
renku/command/schema/composite_plan.py	12	16	75.0%
renku/command/schema/plan.py	11	15	73.33%
renku/core/template/usecase.py	33	37	89.19%

Files with Coverage Reduction	New Missed Lines	%
renku/core/dataset/dataset.py	1	90.95%
renku/core/migration/m_0003__1_jsonld.py	1	87.68%
renku/domain_model/project_context.py	1	96.37%
renku/domain_model/workflow/composite_plan.py	1	81.55%
renku/ui/service/controllers/datasets_add_file.py	1	95.24%
renku/ui/service/errors.py	1	82.18%
renku/ui/service/jobs/contexts.py	1	73.33%
renku/command/rollback.py	2	78.44%
renku/version.py	2	75.0%
renku/command/checks/activities.py	3	74.14%

Totals
Change from base Build 4362983352:	0.2%
Covered Lines:	25582
Relevant Lines:	29511

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💛 - Coveralls

[Panaetius]

Minor comments, but looks good to go