switch build arg to lowercase
# NOTE: this Dockerfile is purely for local development! it is *not* used for
# the official 'concourse/concourse' image.
-ARG BASE_IMAGE=concourse/dev
-FROM ${BASE_IMAGE} AS base
+ARG base_image=concourse/dev
+FROM ${base_image} AS base
# generate keys (with 1024 bits just so they generate faster)
RUN mkdir -p /concourse-keys
