Add content interface to write configurations from another snap

[farshidtz]

Adding the write-slot that was excluded from #158

Closes #299

The snap that writes the config needs to have a corresponding plug, e.g.:

plugs:
  docker:
    interface: content
    content: docker-config
    target: $SNAP_DATA/docker-config

And a logic that modifies the config, e.g. snap/hooks/connect-plug-docker:

# Set log-level to debug
CONFIG_FILE="$SNAP_DATA/docker-config/daemon.json"  
jq '.["log-level"] = "debug"' "$CONFIG_FILE" \
    > "$CONFIG_FILE.tmp" && \
    mv "$CONFIG_FILE.tmp" "$CONFIG_FILE"

Full example: https://github.com/farshidtz/docker-config-provider-snap

Caution

Changes made to daemon.json may be overwritten. The Docker snap with nvidia support writes to the config directory and modifies the daemon.json file during the setup.

[GairePravesh]

Tested, works for me, waiting for customer approval.

[farshidtz]

Thanks @GairePravesh for testing. I'll close this for now because of the configuration conflict with the NVIDIA container runtime setup.

Let's revisit this once there is a demand for it.

[0mniteck]

I will test this soon as well as I have a need for this content slot.
Related issues: #132, #10, #299, #158, #328
Considering trying to build a new generally available docker-credential-helper thats is up to date and multi-arch.
And with a rootless docker daemon (using the config-slot rw)
On the snapstore it will be called simply docker-helper