Documentation: Docker 20.10+ is required to build

[luozhenyu]

Changes

1. Docker 20.10+ is required to build.
   1. Because the --mount option is used without specify experimental dockerfile #syntax directive. https://github.com/adoptium/installer/blob/master/linux/jdk/redhat/src/main/packaging/Dockerfile#L27

   https://docs.docker.com/engine/release-notes/#20100
   buildkit,dockerfile: Support for RUN --mount options without needing to specify experimental dockerfile #syntax directive. dockerfile: update run mounts to stable channel moby/buildkit#1717

2. Remove COMPOSE_DOCKER_CLI_BUILD
   1. Because docker compose is not used.
   2. Because COMPOSE_DOCKER_CLI_BUILD=1 is the default setting. https://docs.docker.com/compose/reference/envvars/#compose_docker_cli_build

[zdtsw]

Sure I agree we should remove unnecessary flag in our code to not cause confusion COMPOSE_DOCKER_CLI_BUILD

As for specify mini docker version, I am not aware of anyone doing local package build.
But if this caused any difficulties, we can definitely have that as a requirement in README

[karianna]

LGTM

[sxa]

LGTM - although mostly for the link syntax fixes :-)