Fix signing Dockerfile

[qbarrand]

Fix the path to the sign-file binary.
Build the signing image in CI.

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[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@4343445). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #324   +/-   ##
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  Coverage        ?   82.92%           
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  Files           ?       26           
  Lines           ?     2753           
  Branches        ?        0           
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  Hits            ?     2283           
  Misses          ?      380           
  Partials        ?       90           

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[ybettan]

/lgtm

[ybettan]

/test ?

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