I have not fitted a solar system yet but can see the argument that the presure release discharge can be discharged into a 5 gal container due to the presence of glycol however due to possible temp of discharge would have thought would have been a metal or high temperature resistant insulated container with maybe view chamber ?
As a gas boiler only system has no such anti freeze ect chemicals,apart from inhibitor,maybe leak sealer and say some mild system cleaner 🙂the discharge pipework should be terminated correctly outside
Another point is when you turn up at a solar instalation and container full of nasty system chemicals,or you have to drain to carry out works,were offically. do you empty the container or system water,do you have to ring up a disposal firm,do you add so many parts tap water to dilute,and test consentrate before you poor down drain ?