What is the correct way to tag the building next to a soccer field that contains the changing rooms and showers? I have look under “sport” and “building” but have not found anything that seems correct.
There is no approved tag for that kind of buildings.
In en-gb these buildings are usually called (sports) pavilions, and there is a decent usage of building=pavilion in OSM (https://taginfo.openstreetmap.org/tags/building=pavilion).
A classic sports pavilion would have sets of changing rooms and showers, storage space for sports equipment (e.g., soccer goalposts in the close season) and maintenance equipment (lawn mowers, rollers etc), and a larger room suitable for events or functions perhaps with a small kitchen. Larger ones (e.g., https://en.wikipedia.org/wiki/Pavilion#/media/File:LeytonCricketGround.JPG would also provide some modest spectating facilities). More recent ones may just have changing rooms on a smaller footprint.