remove legacy build-tags

[thaJeztah]

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.26%. Comparing base (2eec746) to head (f6d49e9).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5904   +/-   ##
=======================================
  Coverage   59.26%   59.26%           
=======================================
  Files         357      357           
  Lines       29771    29771           
=======================================
  Hits        17645    17645           
  Misses      11153    11153           
  Partials      973      973           

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[Benehiko]

!windows is a lot broader than just unix. The package used here is golang.org/x/sys/unix which I assume supports all unix systems, but it's unclear to me if !windows would be too broad for it, wdyt?

[vvoland]

In this case we mostly care about complementing the _windows file, so it's fine.

If someone attempts to build the CLI for such platform, I don't think it makes much difference if they would get an TerminationSignals is undefined vs package unix doesn't provide SIGTERM/SIGINT.

[thaJeztah]

I originally removed the build-tag, but forgot that unix is "special" in Go, because for backward-compatibility, they didn't add support for _unix.go as suffix (as they do for other build-tags, like _windows.go.

I can add a signals_other.go or signals_unsupported.go instead that defines an empty slice instead?

var TerminationSignals = []os.Signal{}

or

var TerminationSignals []os.Signal

@Benehiko let me know what you prefer 👍

[Benehiko]

I think it's fine as is :)

[thaJeztah]

Ah, I also just realised that this is an internal package inside cmd/docker, so really only for building the binary, and not to be used by anyone else, so probably should be fine yes.