Replace xz with zstd

Closes #63 (closed).

Using zstd -T0 -8 instead of gz -T0 -9 results in a larger rootfs file, but requires significantly less time and memory:

zstd  -3 115M   1.60user 0.18system 0:00.32elapsed 557%CPU (0avgtext+0avgdata  130212maxresident)k
zstd  -6 107M   5.03user 0.24system 0:00.72elapsed 729%CPU (0avgtext+0avgdata  149660maxresident)k
zstd  -7 106M   7.33user 0.29system 0:01.04elapsed 728%CPU (0avgtext+0avgdata  174368maxresident)k
zstd  -8 105M   8.45user 0.27system 0:01.23elapsed 707%CPU (0avgtext+0avgdata  173008maxresident)k
zstd  -9 104M   9.35user 0.37system 0:01.42elapsed 683%CPU (0avgtext+0avgdata  335920maxresident)k
zstd -13 104M  30.57user 0.31system 0:04.76elapsed 648%CPU (0avgtext+0avgdata  498740maxresident)k
zstd -19  91M 132.06user 0.53system 0:21.98elapsed 603%CPU (0avgtext+0avgdata 1106328maxresident)k
xz    -9  81M 105.71user 0.58system 0:58.24elapsed 182%CPU (0avgtext+0avgdata 2006964maxresident)k

Additionally this drops bash from the build Dockerfile, since SHELL is not OCI compliant.