fix vikunja docker-compose variables and update github link

[Aaron-McGuire]

What is this PR about?

• Update docker-compose.yml to use the env variables
• Added user 0:0 to avoid permission issues in the vikunja container
• Updated postgres healthcheck
• Remove POSTGRES env variables, use the VIKUNJA ones are already defined
• Update github url in meta.json

Checklist

Before submitting this PR, please make sure that:

• I have read the suggestions in the README.md file https://github.com/Dokploy/templates?tab=readme-ov-file#general-requirements-when-creating-a-template
• I have tested the template in my instance, so the maintainers don't spend time trying to figure out what's wrong.
• I have added tests that demonstrate that my correction works or that my new feature works.

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