restoring data out of docker.img backup

[40foot]

I'm looking for a solution (for non-pros) for the following situation: I just need one configuration file (containing security keys I lost) from a container inside a backup docker.img file (which I can't move or start - too difficult to explain, and its size is 430GB). Is it in any way possible to somehow mount the file system of the docker.img file (if needed), than mount the container file/file system that is inside it and extract the one file out of it, a screenshot of its content would suffice also? If no problem for a pro: Teamviewer session, costs? The container in question: creecros/resourcespace-docker

Edited March 30, 2020 by 40foot
forgot to mention

[Squid]

you can do something like this

mkdir /tmp/docker
mount /mnt/user/system/docker.img /tmp/docker
cd /tmp/docker/btrfs/subvolumes

then you can use the appropriate command lines to search each of the subvolumes that are in there to find your file.  Probably easiest to use midnight commander (mc) at the command prompt to do this.