Add support for argon2-cffi

In the latest [radicale v.3.5.3](https://github.com/Kozea/Radicale/releases/tag/v3.5.3), support for argon2 hashes were added. According to the [updated documentation](https://radicale.org/master.html#htpasswd_encryption): > `argon2` (>= 3.5.3) : This uses an iterated ARGON2 digest of the password with a salt. The installation of argon2-cffi is required for this. So if the docker image could support this, it would support modern password hashes and increase user security.