you identified the package version on ubuntu focal, and try to install this version with a bionic base image. Either do your madison search on bionic or switch to a focal base image.
it is sufficient to run apt-get update once. apt-get install accepts more than a single package.
This will install the latest available nginx packge on a bionic base image:
FROM ubuntu:bionic
ARG NGINX_VERSION=1.14.0-0ubuntu1.7
RUN apt-get update && \
apt-get install -y curl nginx=${NGINX_VERSION}
CMD ["nginx", "-g", "daemon off;"]
I switched the ENV to ARG, as the variable is only required during build time and irrelevant during container runtime.