ci: fix build-docker job to still run when secrets not set/available

[ErikBjare]

[!IMPORTANT] Adds conditional logic to build-docker job in build.yml to control Docker image pushing based on branch or tag.

• Behavior:
  • Adds SHOULD_PUSH environment variable to build-docker job in build.yml to determine if Docker images should be pushed.
  • Conditional execution of Docker login and push steps based on SHOULD_PUSH value.
• Workflow:
  • Modifies Log in to GitHub Container Registry and Push Docker image (base) steps to run only if SHOULD_PUSH is true.
  • Updates push parameter in Build and push Docker image (eval) step to use SHOULD_PUSH.

^{This description was created by for 170e4e4127c88047f180ea0105df154ec791b42e. It will automatically update as commits are pushed.}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 71.58%. Comparing base (66c977c) to head (170e4e4).

:white_check_mark: All tests successful. No failed tests found.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #255 +/- ## ======================================= Coverage 71.58% 71.58% ======================================= Files 61 61 Lines 3952 3952 ======================================= Hits 2829 2829 Misses 1123 1123 ``` | [Flag](https://app.codecov.io/gh/ErikBjare/gptme/pull/255/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Erik+Bj%C3%A4reholt) | Coverage Δ | | |---|---|---| | [anthropic/claude-3-haiku-20240307](https://app.codecov.io/gh/ErikBjare/gptme/pull/255/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Erik+Bj%C3%A4reholt) | `70.62% <ø> (ø)` | | | [openai/gpt-4o-mini](https://app.codecov.io/gh/ErikBjare/gptme/pull/255/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Erik+Bj%C3%A4reholt) | `70.52% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Erik+Bj%C3%A4reholt#carryforward-flags-in-the-pull-request-comment) to find out more.

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