ci: Introduce "Build Container (BC)" type

To comply with security requirements, we must rebuild the benchmark
containers regularly. We will do this using a gitlab scheduled pipeline.
To prepare for that, let's add the concept of a pipeline type via the
PIPELINE_TYPE variable. The existing type is now called "Continuous
Integration (CI)" and we introduce "Build Container (BC)", which when
invoked, will build a specified container tagged with the specified
version.

The scheduled pipelines will be setup to invoke this new type.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>