Germany does not have extraterritorial prison colonies.
I think this is a combination of hyperbole and choosing the wrong word. Germany does have housing for refugees and asylum seekers while they wait for asylum to be granted or denied, and the conditions there might be construed as prison-like depending on your political point of view. Technically it's not a prison.
Asylum seekers are not permitted to do paid work, for example, and I think they also cannot exercise freedom of movement inside the EU or Germany (they have to clear moving to a different city with the authorities first and they are generally not given enough money to actually move around).
The EU has made a deal with Turkey to prevent refugees from Syria to come to the EU. That might be construed as an extraterritorial prison camp (the press has been mostly silent about the conditions those people are held in but I assume they are not very nice). Technically, even if that is a prison colony, it's not Germany's.
Greece has also held refugees in camps, one could interpret this as "on behalf of Germany", which I think is technically incorrect but in a sense still true.
German political debate has unfortunately become a shouting match between leftists who use war-cry terminology like OP did here, and right-wing populists who complain that asylum seekers should be sent home straight away and should not be allowed a legal way to protest if they are denied asylum status.
As always in shouting matches and war, the first victim are the facts and the truth.
Personally I think it is abhorrent how the EU is building a fortress to keep refugees out. Compared to most other EU countries however, Germany is by far not the worst of the bunch. Also I think far too little is being done to remove the reasons why people would leave everything behind and flee to the EU. You can't win scenarios like this by fighting the symptoms and doing cosmetics only.
Also, dear American friends, the biggest reasons for refugees from the middle east fleeing to Europe are a) your constant wars and meddling in the area and b) climate change, which from Europe also looks like it is disproportionally being caused by you.
(Prepare for downvotes in 3... 2...)