Add check for required Linux capabilities

[DL6ER]

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Add check for required Linux capabilities. Complain loudly when the required capabilities are missing.
Initially, I wanted the check to be fatal, but I know there are many users out there that run pihole-FTL on marginally supported platforms which (partially) don't even support capabilities. Since I don't want them to be unable to start FTL at all, we only log missing capabilities instead of failing hard.

Fixes #432

This template was created based on the work of udemy-dl.

[AzureMarker]

This also updates the docker build containers with the libcap-dev dependency.