modernleft-docs/Docker Images/quartz/use xxd in docker.md

# on host
docker run --rm -it alpine:latest sh

# in container
apk add xxd openssl

# everything after this is one command
echo PDS_PLC_ROTATION_KEY_K256_PRIVATE_KEY_HEX: $(openssl ecparam --name secp256k1 --genkey --noout --outform DER | tail -c +8 | head -c 32 | xxd --plain --cols 32)