Dockerfile: Add libgnutls package for building mkeficapsule command

For adding signing feature for capsule authentication to the host tool,
mkeficapsule, we will link gnutls library for crypto operation.
Since we need this command to complete the capsule authentication test
on sandbox in CI loop, necessary packages must be installed on the host.

See my patch, "tools: mkeficapsule: add firmware image signing."

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>