Bump arm64 Debian Dockerfile to use latest armhf if available

[phantomski77]

I've modified the arm64 Dockerfile, so instead of relying directly on the latest arm64 versions in the official Debian and/or ntop.org repository from https://packages.ntop.org/apt-stable/buster/all/, it adds the Raspberry Pi http://packages.ntop.org/RaspberryPI/ repository and then adds armhf architecture to Debian's dpkg package tool, so it installs armhf versions where available.

ntopng

• the latest arm64 version is only 3.8.190204
• the latest armhf version is 4.3.210711

ntopng-data

• the latest arm64 version is 4.2.210629, but ntopng 3.8 requires 3.8+dfsg1-2.1
• the latest armhf version is 4.3.210629

Using latest versions (available only in armhf) needed few modifications:

• armhf architecture needs to be added to apt package manager as possible source, otherwise ntopng won't install (not available in all repository for Raspberry Pi)
• ntopng needs to be forced to install with armhf specified, otherwise it looks for arm64 which is not available
• it then installs its dependencies as armhf also, if required (as dpkg recognises additional architecture)
• libcap2:armhf and libzstd1:armhf were missing in dependencies, but required, so they're installed manually

[lucaderi]

Thank you