fix: docker build

[zentale]

Build was failing with this error:

Installing hiredis 0.6.0 with native extensions

NoMethodError: undefined method `dir_mode=' for "/usr/local/bundle/cache/hiredis-0.6.0.gem":String

    @package.dir_mode = options[:dir_mode]
            ^^^^^^^^^^^

[pschultz]

Leaving this here so it shows up in the search:

Our build failed with a different error message, but the fix is the same. Our error was:

#7 27.81 Error details
#7 27.81
#7 27.81     NoMethodError: undefined method `inflate' for Gem:Module