chore: fix Dockerfile go mod download step

[markphelps]

Fixes error when building local Dockerfile:

 => CACHED [build 6/9] COPY go.sum .                                                                                                  0.0s
 => ERROR [build 7/9] RUN go mod download                                                                                            13.2s
------
 > [build 7/9] RUN go mod download:
#15 13.15 go: go.flipt.io/flipt/errors@v1.19.3 (replaced by ./errors/): reading errors/go.mod: open /home/flipt/errors/go.mod: no such file or directory

[codecov]

Codecov Report

Merging #1497 (34a5d2a) into main (a908fdb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1497   +/-   ##
=======================================
  Coverage   77.62%   77.62%           
=======================================
  Files          46       46           
  Lines        3611     3611           
=======================================
  Hits         2803     2803           
  Misses        643      643           
  Partials      165      165           

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