Support for docker commands within builds about earthly

[vladaionescu]

I suspect adding support for [btrfs] is not too difficult, as it's built on top of containerd & docker which have support for btrfs.

Seems to indeed be the case.

[vladaionescu]

FTR, neither ZFS nor BTRFS helped us here. The setup seems to be very brittle - we were getting a lot of fs consistency issues.

Next up, we'll try this approach:

• Use a host bind-mounted dir for docker-in-docker uses. Within it, create sub-dirs for each docker daemon used within the build. Pass this bind-mount all the way within the build - and within it, mount the docker dir (/var/lib/docker) of the docker-in-docker daemon. Thus, the daemon can now use a native partition (eg ext4), which allows use of overlayfs on top.

[vladaionescu]

This is now complete as of #230