ARG argument on Dockerfile seems to be not needed

[jordigarcl]

I believemkdocs does not make special use of this environment variable.

getting-started/Dockerfile

Line 30 in 5bb26cc

ARG LANGUAGE

[mikesir87]

Ah! Good catch. This is an artifact from where this repo came from (dockersamples/101-tutorial), which was being translated into multiple languages. We decided to skip that (for now anyways), but that ARG snuck through.