the minimum proof_of_work_bits value on-chain is 30

the previous implementation does not reliably iterate through a hashmap in the desired order. this fix ensures that the hashmap is iterated like an array, which is what we want.

- Moved Dockerfile from ./examples/bootloader to top-most directory and merged it with the existing Dockerfile (this allows calling the rust code in the same Dockerfile and essentially makes running the demo easier)
- Removed Makefile since it's no longer needed
- Updated README accordingly