I used to think I was fine with tiny living, because my entire childhood home was 20 sqr meters, but I now live alone in such a space and it's too small. I have huge windows, but the walls are closing in. The space is too small to furnish in a way which wouldn't hinder an aspect of everyday life. I'm so tired of algorithm trying to convince me tiny living is fine when it shows even worse conditions than mine. I wish I lived in 30 sqr meters or so. 40 sqr meters are way too burgoise for me, but I could use extra 10 sqr meters solely for the purpose of farting.
Best you might be able to do is take the shipping container and build a more standard construction around it, cause holy carp! a those things become ovens in the summer and fridges in the winter.