fix: invalidate railpack build cache when env changes

[rnkp755]

What is this PR about?

Fix Railpack application rebuilds so environment variable changes are reflected without requiring Clean Cache or Docker cache pruning.

Railpack builds pass application env vars as BuildKit secrets, but BuildKit secrets do not reliably invalidate cached layers by themselves. Previously, Dokploy only passed the secrets-hash build arg when cleanCache was enabled, so regular rebuilds could reuse stale cached output after env vars changed.

This PR always passes a hash of the resolved environment variables to the Railpack Docker build. The normal build cache is still used when env vars do not change, while env changes now invalidate the relevant cached build output.

Checklist

Before submitting this PR, please make sure that:

• You created a dedicated branch based on the canary branch.
• You have read the suggestions in the CONTRIBUTING.md file https://github.com/Dokploy/dokploy/blob/canary/CONTRIBUTING.md#pull-request
• You have tested this PR in your local instance. If you have not tested it yet, please do so before submitting. This helps avoid wasting maintainers' time reviewing code that has not been verified by you.

Issues related (if applicable)

closes #4516