how to set base url for thiea docker image
AFAIK this is not possible at all with Theia to change base URL.
You can use nginx for reverse proxy.
You could manipulate index.html and insert base tag with relative href after bundling the app. Or use a custom webpack configuartion with html plugin if you index html to be created with such tag.
and what is the reason bheind this error
sorry, this image is unreadable