install subcommand writes directly to a block device (disk) and consumes the entire device. The device specified to the installer needs to be available and not currently in use. You cannot target a disk that is currently mounted.
Build coreos-installer and use it to install a Fedora CoreOS
testing image to a partitionable loop device:
cargo build truncate -s 8G image-file sudo losetup -P /dev/loop0 image-file sudo target/debug/coreos-installer install /dev/loop0 -s testing
Releases can be performed by creating a new release ticket and following the steps in the checklist there.