How to restrict SSH /SFTP User to a sub folder of an apache website?

SSH has the capability to chroot a user built-in (since 4.9p1), but using it for interactive sessions is somewhat difficult.

Restricting sftp is fairly easy though, and can be configured on a per user, or per group basis

Match User joeblogs
    ChrootDirectory /data/htdocs/mywebsite/store/
    ForceCommand internal-sftp
    AllowTcpForwarding no

Interactive sessions are more complex, because the user needs a shell, and some device files to be present.

From man 5 sshd_config:

The ChrootDirectory must contain the necessary files and directories to support the user's session. For an interactive session this requires at least a shell, typically sh(1), and basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), arandom(4) and tty(4) devices

I wouldn't recommend this, as you would need to build and maintain a copy of any utilities they need within their chroot. If you still want to try going this route, you can read more about it the sshd_config man page, and here