Switch container image to nginx-unprivileged

[Lykos153]

This is another take on #17927 which was reverted in #17990. I'm approaching this by using the alpine version of the [official nginx-unprivileged image]

Fixes #25926

(https://hub.docker.com/r/nginxinc/nginx-unprivileged) which uses port 8080 by default.

Why use an unprivileged container?

• It reduces attack surface. Though isolated by namespaces, being root in a container is still more powerful than being user in a container.
• As commented in Don't run nginx as root in docker #17927, the container as it is already drops root privileges after opening the TCP socket. However, some environments (eg. OpenShift) don't allow root containers to be run at all, even if they drop privileges afterwards.
• Unprivileged containers can be more complicated to handle when dealing with persistent volumes. As this container only serves static content, this isn't an issue and there is no reason to run it as root.

Changes in this PR:

• Base container image on nginxinc/nginx-unprivileged
• Move document root to /app
• Switch port to 8080
• Update the commands in the documentation so the result is the same as before

I understand that there might be concern that changing the container port could break existing deployments. I can see multiple ways to deal with this:

• Bump the version in a way that indicates incompatibility. Though, as the tags mirror the app version, this is probably not feasible.
• Build two versions of the image, so we have eg. v1.9.9 and v1.9.9-unprivileged. Optionally, there could be a deprecation phase after which there won't be any root-versions anymore.
• Create an image that can handle being run as root or as a user. This image would by default run as root and provide the service on port 80, but if run as a user, would not fail but use a configuration that uses port 8080.

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[turt2live]

This looks fine, though the port change means we need to communicate this in advance. Will talk to the app team about this.

[ibotty]

Any news? This would make it possible to run in constrainted environments (e.g. OpenShift).

[t3chguy]

According to the image docs

The default NGINX listen port is now 8080 instead of 80 (this is no longer necessary as of Docker 20.03 but it's still required in other container runtimes).

We can revert back to port 80 to minimise the migration risk or provide two versions of the image as per your suggestion

Build two versions of the image, so we have eg. v1.9.9 and v1.9.9-unprivileged. Optionally, there could be a deprecation phase after which there won't be any root-versions anymore.

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