README fixes, Dockerfile bump

scripts/jsonnet/Dockerfile

@@ -1,9 +1,8 @@
-FROM golang:1.11-stretch
+FROM golang:1.12-stretch
 
 ENV JSONNET_VERSION 0.12.1
 
-RUN apt-get update -y && apt-get install -y g++ make git jq && \
-    rm -rf /var/lib/apt/lists/*
+RUN apt-get update -y && apt-get install -y jq && rm -rf /var/lib/apt/lists/*
 RUN curl -Lso - https://github.com/google/jsonnet/archive/v${JSONNET_VERSION}.tar.gz | \
     tar xfz - -C /tmp && \
     cd /tmp/jsonnet-${JSONNET_VERSION} && \