allow dynamic ip resolve on php handler

[SebastianRzk]

Problem:

When using upstream in the nginx config, the ip for app is resolved once. So if the app container gets updated (e.g. through watchtower, the ip changes and the server php-handler is not reachable anymore.

Fix:

• Add docker dns as resovler
• replace upstream with variable (set), to trigger the dns lookup

Source / Inspiration:

bug on the watchtower github

[J0WI]

We use to copy the example from https://github.com/nextcloud/documentation/blob/master/admin_manual/installation/nginx-root.conf.sample and apply fixes directly in the upstream documentation.

This is an edge case as it affects only the very specific case with nginx and backends running as container but it's not specific to Nextcloud. The examples are not meant to be production ready configurations but they make it easier to develop a custom configuration. I'm undecided whether it's worth to cover this in the examples or just refer to the documentation and examples of nginx.