If that's true what happens when the volume of a container,
'containing' a vacuum, is increased? Is space literally expanding in
No, for the reason that the metric of space hasn't changed when the container volume is increased, i.e., the measured distance between two points within the container is the same before and after container volume increases.
This isn't the case for the metric expansion of space:
The expansion of the universe is the increase of the distance between
two distant parts of the universe with time. It is an intrinsic
expansion whereby the scale of space itself changes. The universe
does not expand "into" anything and does not require space to exist
"outside" it. Technically, neither space nor objects in space move.
Instead it is the metric governing the size and geometry of spacetime itself that changes in scale.