more bugfixes & clarity in readme

- Better info about $DATA_VOLUME and $SFTP_CHROOT in README.md
- Smaller fixes and refinements to README.md
- Fixed wrong order of events when setting $SFTP_CHROOT
- Fixed setgid not being set on the shared chroot/files
- Fixed password env var being kept after setup, which is just a silly risk
- Split up user and chroot setup, preventing failure upon user conflict
fixed messing up /etc/ssh symlinking
refined docs & changed $DATA_VOLUME

- Refined and improved both banner and README.md, including update to reflect any changes
- Renamed $DATA_VOLUME to now more appropriate $SFTP_DATA_DIR
- Changed default for $SFTP_DATA_DIR to /data/sftp, minimizing both typing and conflicts in data storage
README.md markdown fix

- Fixed strong emphasis nested in italic
- Worked around some docker.io markdown issues
fixed obvious typo in $SFTP_DATA_DIR default value
finally fixed and no new issues

- Finally fixed everything related to $SFTP_CHROOT
- Deprecated the manual setting of $SFTP_CHROOT (pointless & messy)
- Tested all supported configurations this time, now it should work in all cases!
missed a $SFTP_CHROOT occurence in README.md