fix(dockerignore): trim trailing whitespace

[kalioz]

Remove the trailing whitespaces in dockerignore files, allowing the following structure to be conform to the official docker build.

*
! files
! src

fixes #2732

[kalioz]

Jenkins builds crash because of reasons outside of this PR 😥 I'll try to re-trigger it tomorrow, unless a kind soul can trigger it directly on jenkins without pushing to git :)

[milas]

Thanks for your PR! And you're absolutely correct, our docs do say:

A preprocessing step removes leading and trailing whitespace and eliminates . and .. elements using Go’s filepath.Clean. Lines that are blank after preprocessing are ignored.

(emphasis mine)