[BUG] Cannot change backend URL when using docker image

**Describe the bug**
The `VITE_BACKEND_URL` environment variable is evaluated at compile time, meaning it cannot be set unless the docker image is manually built in an environment with the environment set.

**To Reproduce**
1. Run `docker run --rm -p 3024:80 -e VITE_BACKEND_URL=https://example.com --name drawdb-test ghcr.io/drawdb-io/drawdb:latest`
2. Navigate to `http://localhost:3024/editor/`
3. Open browser dev-tools
4. Click on Share
5. Notice that it tries to make a POST request to `http://localhost:3024/editor/diagrams/undefined/gists` (undefined because VITE_BACKEND_URL resolved to undefined during build-time)

**Expected behavior**
VITE_BACKEND_URL should be correctly evaluated (in this case `https://example.com`) instead of being evaluated as `undefined` or as whatever value existed for it during compile-time

**Desktop (please complete the following information):**
- OS: GNU/Linux
- Browser Floorp 12.14.2@151.0

**Additional context**
`import.meta.env.VITE_BACKEND_URL` is evaluated at build-time.
https://vite.dev/guide/env-and-mode#node-env-and-modes

Start by tracing where import.meta.env.VITE_BACKEND_URL is consumed and how the Docker image starts in the deployed container. Reproduce the issue with the provided docker run command, then verify that the Share request uses the runtime-supplied https://example.com URL rather than a compile-time or undefined value.

Written by the indexing model from the issue text.