Validation fixes

[mpsonntag]

This PR fixes various issues when running the validation variants:

• when a NIX file cannot be opened, the occurrence is logged as a validation error and the validation continues with the next NIX file. Closes [NIX validation] Broken NIX file leads to validation abort #106.
• provide the appropriate error when the BIDS validator is not run from the root BIDS folder structure. Closes [BIDS validation] Make failure on non-bids repository less non-descript #107.
• when running a public validation, the stored result is now stored using the actual HEAD commit hash.

Further the PR

• upgrades the Docker "Runner image" version to alpine:3.16
• upgrades and simplifies the Docker nixio build

[coveralls]

Pull Request Test Coverage Report for Build 2761444102

• 35 of 85 (41.18%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+1.2%) to 37.882%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/web/ginutils.go	0	11	0.0%
internal/web/validate.go	35	74	47.3%

Files with Coverage Reduction	New Missed Lines	%
internal/web/validate.go	1	47.47%

Totals
Change from base Build 2760462758:	1.2%
Covered Lines:	719
Relevant Lines:	1898

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💛 - Coveralls