Google Translate says the writing is: Bunker christmas The shelving and mirror may indicate it is not a concrete bunker, hammering nails into concrete is a little difficult. Sparse food, what appear to be two apples. Plus that odd shape in the top right corner.
I agree that hammering nails into concrete is difficult. Many of the field bunkers were made out of logs and earth, but this one looks like it is wood sheeting. If they did what we did in the field we partitioned areas off inside our bunkers with wood sheeting for opsec and officer comfort. Couldn't of been that sparse, there is a guy with a beer. The odd shape looks like headphones. Could they be tanker or vehicle headphones for the officer?