fix nginx permissions in docker image

[jschaul]

This fixes the error seen on our kubernetes deployments:

Starting nginx
Setting up watches for /etc/wire/nginz/upstreams
nginx PID: 9
Setting up watches.
Watches established.
2020/02/19 08:22:55 [emerg] 10#0: mkdir() "/var/cache/nginx/client_temp" failed (2: No such file or directory)
nginx: [emerg] mkdir() "/var/cache/nginx/client_temp" failed (2: No such file or directory)

[jschaul]

Manually tested to fix the issue by doing the following:

make docker
docker tag quay.io/wire/nginz:local quay.io/wire/nginz:2.73.15
docker push quay.io/wire/nginz:2.73.15

which overrides the broken build.