Most supermarkets have bins that are accessible without having to break a lock or jump a fence. You can just search the term "Supermarkt" at google maps and arrive one hour after closing time.
Dumpster Diving Opportunities
Grünschnabel Biomarkt in Oldesloer Straße 18, closes at 6pm. They have brown containers on the left side of the building filled with some fruits and vegetables
ALDI Bad Segeberg in Eutiner Straße 24, closes at 9pm. They have one green container on the right side of the building.
NETTO - Nur echt mit dem Scottie their dumpster is in the street "Wischhof" and they close at 9pm.
Netto Filiale in Ziegelstraße 115, closes at 10pm. Their dumpster is on the right side of the building.
Nahkauf Supermarkt in Glindenberg 3, closes at 9pm. Dumpster on the right side of the entrance.
Lidl in Lindhofstraße 14, closes at 9pm. To enter their dumpsters you need to open a lock with a triangular socket or pliers with thin claws. But it's not necessary even going there as there are enough other options.
Food Market on Wednesdays and Saturdays around "Am Markt 1", not sure when they close (around 2pm?) as I haven't tried them but I guess Saturday would be more successful.
Bakeries : Most bakeries say they support the "Tafel" which is an organization providing mostly food to people in need. Some have to return their bread to the headquarter or give it to animals. I praise them when hearing they support the Tafel but say that it would be nicer if humans could enjoy the bread instead of supporting the animal industry.
Good luck and have fun :)