Add json file permissions to healthcheck

[vdrover]

URL to the Wiki page

https://github.com/qdm12/gluetun-wiki/blob/main/faq/healthcheck.md

What's missing?

I had a healthcheck issue that was resulting in this error: ERROR reading servers from file: decoding servers: invalid character '\x00' in string literal

I noticed my servers/json file had switched permissions from my user, to root. This was a problem as my gluetun runs as m ubuntu user, not root.

I would add this error to the list of potential problems (specifying linux users perhaps), and propose fixing permissions and restarting the container.