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

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Project coverage is 59.69%. Comparing base (ef50c1f) to head (c391ca0).
Report is 28 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/imageproxy/main.go	0.00%	13 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (ef50c1f) and HEAD (c391ca0). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (ef50c1f)	HEAD (c391ca0)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #255       +/-   ##
===========================================
- Coverage   85.21%   59.69%   -25.53%     
===========================================
  Files           6       12        +6     
  Lines         717     1228      +511     
===========================================
+ Hits          611      733      +122     
- Misses         76      463      +387     
- Partials       30       32        +2     

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[albb0920]

@googlebot I signed it!

[willnorris]

thanks for this. I reworked it a tad, since things have changed a bit since you opened this, but looks good.