fix: prevent error if "dockerfile" isn't set

[dunglas]

Closes #1916. The problem was that the default Dockerfile name isn't taken into account if there are several lines in the matrix.

Thanks for this great project.

[vercel]

@dunglas is attempting to deploy a commit to the Goreleaser Team on Vercel.

A member of the Team first needs to authorize it.

[caarlos0]

hey, thanks for the PR :)

can you to add/fix/change tests too?

[codecov]

Codecov Report

Merging #1917 (5db68e3) into master (5054769) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1917   +/-   ##
=======================================
  Coverage   81.73%   81.73%           
=======================================
  Files          73       73           
  Lines        3866     3866           
=======================================
  Hits         3160     3160           
  Misses        580      580           
  Partials      126      126           

Impacted Files	Coverage Δ
internal/pipe/docker/docker.go	91.78% <100.00%> (ø)

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[dunglas]

@caarlos0 done!

[caarlos0]

awesome, thanks @dunglas 🚀

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.