Edit: ops, that joke wasn’t clear.
A prisoner’s uniform needs to be cheap, distinctive, and easy to spot it doesn’t need to be clean.
If the person were a prisoner he wouldn't be carrying a rifle..
Thus the joke…
It’s a play on words, and the involuntary nature of service in the German military at the time.
German Army had 1.3 million conscripts and 2.4 million volunteers in the period 1935-1939 so odds are he signed up to be there.
Those numbers aren’t independent of each other. People about to be drafted will often volunteer to be in a military as a volunteer rather than a draftee, to get the waiting over with, etc.
For some value of 'joke'.
Fair, but it’s a meme thread. My initial thought was.
Pioneer: O panzer of the lake, why are our uniforms white? Panther: They must be easy to spot.
But, I tried to reach past the pun and failed.