In Germany (also in Berlin), there are a lot Places for Self Service Washing, using a High Pressure Steam Cleaner for which you have to pay by Minutes.
As there is no other water suply, I guess you will have to use this to get Water for your Buckets and Foam.
Apart from this, in general, it is not legal in Germany to wash a Car on the Street or even in private Places without having a Oil separation Plant in Place.
I wonder, if this is not also the Case in the Netherlands...