Docker volume mount into public folder gives Content_Length_Mismatch error

[mouldt]

I have a static file that I want to load into the client side at startup e.g.test.json. I have a plugin on the app:beforemount hook in Nuxt.
I place my file in the public folder and I can therefore load the file into the app e.g. fetch('/test.json')
This all works in dev. I am putting my nuxt app into a container. With the build container I want to be able to change test.json when I run the container. So I use docker run -v to replace the 'test.json' inside the container with one I supply. Now when my client app calls fetch('/test.json') its does get the replaced content but the content-length is incorrect and hence i get a Content_Length_Mismatch error in the browser.
It seems, from the docs, the content-length is coming from meta data ascertained at build time. Is there any way to override this behaviour so the content-length is correctly set at runtime from the content rather than from the metadata?