docker health check for disk space not working as intended

I've got an nginx container which ends up with a full disk after it's been running for about 10 days. So if a new version of the app isn't released, errors start to occur that look like;

2022/01/15 22:45:04 [crit] 13#13: *406812 mkdir() "/var/cache/nginx/uwsgi_temp/9/07" failed (28: No space left on device) while reading upstream...

2022/01/15 22:44:37 [crit] 13#13: *406820 pwritev() "/var/cache/nginx/client_temp/0000001078" failed (28: No space left on device)...

This happened over the Christmas break so I thought the ideal situation here is to have the container health check ensure that there is free disk space. I thought I had achieved that with this container setup (but clearly not);

FROM nginx:1.21.5-alpine-perl

RUN apk update && \
    apk add --no-cache dnsmasq supervisor curl

COPY ./config/supervisord.conf /etc/supervisor/conf.d/supervisord.conf
COPY ./config/nginx.conf /etc/nginx/nginx.conf

HEALTHCHECK --interval=15s --timeout=30s \
    CMD exit $(( $(df / | tail -n1 | awk '{print $5}' | sed 's/\%//') > 95 ? 1 : 0 )) || exit 1

How should I check for disk space in the health check?

You should check your nginx configuration and make sure its cache size limits are configured properly according to the space you have available in the container.

Enter the container and check which directory inside it occupies the space using du (my first guess would be log files). Check if you can change the nginx configuration in a way that the container doesn't fill up the filesystem. If not, mount an external volume in the problematic directory.