The ZFS 0.8 has dependency on libcrypto.so.1.1 which in turn
requires GLIBC_2.25 supported by the system. Changed the docker
image to 18:04 as 16:04 has glibc version 2.23.
Also updated the README with the supported system details.
Signed-off-by: Pawan <pawan@mayadata.io>