Dockerfile for dev container updates (macOS)

Enhance docker image script start-build-env.sh for macOS:

1. update to use dscl instead of getent on MacOS in getting gid for "docker" group
2. change from HOME to DOCKER_HOME_DIR:

                              RUN chown -R ${USER_NAME}:${GROUP_ID} ${DOCKER_HOME_DIR}/.cache

ENV GOPATH ${DOCKER_HOME_DIR}/beam/sdks/go/examples/.gogradle/project_gopath