feat: custom base path

[pwuersch]

This adds a custom base path option to the Dockerfile. By specifying the BASE_PATH build argument when building the Dockerfile, the Vite build step will use this path as the root for all assets and generated bundles.

By default, the root will remain as /, which is the same as now. The only minor change is the assumption of Bash variables in the excalidraw-app/package.json's build:app:docker script, but because I assume it's mostly used for building with Docker, that's a safe assumption to make.

[JannikStreek]

Thanks for the PR! Just to understand this better, what is the use case to configure this?

[pwuersch]

Thanks for the PR! Just to understand this better, what is the use case to configure this?

We are deploying this application on a subpath like example.com/apps/excalidraw. For the frontend to load the assets accordingly, the Vite build step needs to be set to the correct URL. By default, this Excalidraw's Vite configuration assumes all the files are relative to the root /. This then breaks our use case because, e.g., our generated Excalidraw assets are actually located at example.com/apps/excalidraw/assets.

With this change, the BASE_PATH build arg can be set to (in our case) /apps/excalidraw during the container build, which adds this prefix to all resource URLs in places like the generated index.html. This way, the URLs end up as /apps/excalidraw/assets/….

This does not change the current behavior for anyone omitting the BASE_PATH container build argument because of its default value of /, which is in line with the Vite defaults used in this project.