Fix JSON gem dependency error when using '--json' option with docker (#408) (#409)
Fix JSON gem dependency error when using '--json' option with docker (#408) (#409)

By installing Alpine's 'ruby-full' package, all the MDL dependencies with the
ruby standard library are resolved (including not only 'json', but also 'etc'
gems).

Signed-off-by: Pablo Orviz <4b8548a9b1497c49a5e468d2020746e3ebafad8f@ifca.unican.es>
markdownlint/markdownlint