Upload dockerd configuration over HTTP

[killenheladagen]

Let admin upload daemon.json over HTTP, enabling control over those part of the dockerd configuration that are not exposed through parameters today.

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[killenheladagen]

This is a nice feature but it is not needed to get rootless Docker delivered. In addition it adds a new dependency on the fastcgi setup meaning that that would be harder to remove if we find a better solution for the TLS upload It is also not clear how we explain to the user (or handle) if the file contains settings that we already use as inputs to dockerd.

I would prefer that you do not merge this.

The main use case for this is to allow ourselves to upload proxy settings without SSH. If we would switch to a different solution for TLS upload, I assume that solution would also work for daemon.json.

If a configuration is specified both in daemon.json and on the command line, dockerd will not start. Perhaps we need consider this even for the current way we expose daemon.json. We could add a note in the documentation about which entries in this file that are not configurable.

I find it odd that we provide a mechanism for per-device configuration of dockerd but we don't want to make it as convenient as possible to use, even for ourselves.

An alternative could be to provide the proxy settings as AXParameters, or if there are already other proxy settings in the device that we can use.