dockerfile-template: install packages to support ssh-server

There is a need to recover code coverage logs from FVP containers [1] so
install required packages. Also, use docker ENV variables in the
dockerfile-template to hold packages dependencies, making it
cleaner the package installation phase.

[1] https://review.trustedfirmware.org/c/ci/tf-a-ci-scripts/+/9632

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: Iafb730c26412954026221be54d39081aa7dd2e26
3 files changed