feat: support custom Docker image via DOKPLOY_IMAGE env var

[vtomasr5]

What is this PR about?

Add support for running Dokploy with a custom Docker image from any registry. This allows users to deploy modified Dokploy images from private registries without the UI overwriting them during reload/update.

The DOKPLOY_IMAGE environment variable accepts a complete image path (e.g., myregistry.io/my-dokploy:v1.0) and takes priority over RELEASE_TAG.

Priority order:

1. DOKPLOY_IMAGE (full custom image path)
2. RELEASE_TAG (custom tag with official image)
3. Default: dokploy/dokploy:latest

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.

Issues related (if applicable)

Screenshots (if applicable)