Enable DNS over HTTPS support (dnsdist) + other misc. stuff

[Zaczero]

Closes #16.

Key changes:

• Enabled DoH support
• Improved readability
• Disabled GPG key auto-retrieval
• ~Set explicit UID/GID~ Removed dnsdist user/group creation

Compatibility issue:

~The current explicit UID/GID (1000:1000) differs from the default one (101:101), which will cause permission issues for people running the non-root user (which is not a default behavior - affected user base is minimal). However, using implicit UID/GIDs is not reliable in the first place, so I suppose the change does more good than harm.~

No longer related, read the comment below.

[Zaczero]

One more change, feel free to discuss if my understanding is wrong:

Remove user/group creation (it's useless)

Currently the UID/GID is optionally set by the host, and it can be set to any arbitrary value (not just 1000). Since 1. the container itself makes no use of the created user/group and 2. the dnsdist packages does not require a home directory, there is really no point of creating the user/group. It would make sense if we were to forcefully downgrade the user by using docker's USER command - but we don't. By default package runs as root and the user can optionally change the UID/GID by setting proper docker arguments.

The change should be fully backwards compatible, even for the users who run the package as a non-root.