Older versions of git included in older linux distributions are not able
to download source from Github. A newer version is required with a newer
OpenSSL. This requires to also build curl with the same OpenSSL.
CMake is downloaded precompiled if available (64bits system) or compiled
from source otherwise.
Added SSH support to enable Git checkouts that use SSH instead of
HTTP(S).
The SSH directory is assumed to be in $HOME/.ssh; however:
export SSH_DIR=/my/custom/dir
will override the setting and allow for configurable settings.
$SSH_DIR is then mounted as host-volume in the Docker container and
placed in /home/<user>/.ssh
