refactor: dockerfile adjustments, add lefthook and megalinter

[nightah]

This change adjusts the Dockerfile with enhancements only available in newer Docker and Buildx releases.

I have also added LeftHook and MegaLinter with an appropriately adjusted configuration for linting.

MegaLinter is using the beta variant because of this issue.

I've also renamed shell scripts to include the .sh extension so editorconfig and linting changes apply accordingly.

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	1		0	0	0.18s
✅ BASH	bash-exec	4		0	0	0.01s
✅ BASH	shellcheck	4		0	0	0.05s
✅ BASH	shfmt	4	0	0	0	0.01s
✅ DOCKERFILE	hadolint	1		0	0	0.26s
✅ EDITORCONFIG	editorconfig-checker	17		0	0	0.21s
✅ MARKDOWN	markdown-link-check	1		0	0	0.99s
✅ MARKDOWN	markdown-table-formatter	1	0	0	0	0.17s
✅ YAML	prettier	4	0	0	0	0.32s
✅ YAML	yamllint	4		0	0	0.26s

See detailed report in MegaLinter reports
Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff