Add support for UNIX domain socket

[albb0920]

This PR adds UNIX domain socket support for the addr option.

I want this as I run all my services behind nginx with unix domain socket.
I suppose there are more reasons someone would want to run
imageproxy without listening to a TCP port.

I never wrote any Go before this PR, I hope I haven't done anything stupid...

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[codecov]

Codecov Report

Merging #255 (bbb9aca) into main (c08b3c5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #255   +/-   ##
=======================================
  Coverage   87.27%   87.27%           
=======================================
  Files           6        6           
  Lines         503      503           
=======================================
  Hits          439      439           
  Misses         36       36           
  Partials       28       28           

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[albb0920]

@googlebot I signed it!