Stacker has a few a runtime dependency as well, namely, umoci. You can
install it with:
go install github.com/openSUSE/umoci/cmd/umoci
To use unprivileged stacker, you will need a kernel with user namespaces enabled (>= 3.10). However, many features related to user namespaces have landed since then, so it is best to use the most up to date kernel. For example user namespaced file capabilities were introduced in kernel commit 8db6c34f1db, which landed in 4.14-rc1. Stock rhel/centos images use file capabilities to avoid making executables like ping setuid, and so unprivileged stacker will need a >= 4.14 kernel to work with these images. Fortunately, the Ubuntu kernels have these patches backported, so any ubuntu >= 16.04 will work.
If you are running in a btrfs filesystem, nothing needs to be done.
If you are running in a non-btrfs filesystem, but as root, then stacker will automatically create and mount a loopback btrfs to use.
If you are running as non-root in a non-btrfs filesystem, then you need to prepare by, with privilege, mounting a btrfs under "./roots" first. You can see this being done in tests/main.sh:
truncate -s 100G btrfs.loop
mkfs.btrfs btrfs.loop
mkdir -p roots
sudo mount -o loop,user_subvol_rm_allowed btrfs.loop roots
sudo chown -R $(id -u):$(id -g) roots