Debian with nginx docker image won't update - docker

Question

I'm bulding nginx in a Debian-based docker image. Every time I run it, it shows me the current nginx version nginx/1.10.3. I need it to download the latest stable nginx.
This is my Dockerfile:
FROM debian:latest
RUN apt-get -y update
RUN apt-get install -yq gnupg2
RUN apt-get install -yq software-properties-common
RUN apt-get install -yq lsb-release
RUN apt-get install -yq curl
RUN add-apt-repository "deb http://archive.canonical.com/ $(lsb_release -sc) partner"
RUN add-apt-repository "deb http://nginx.org/packages/debian `lsb_release -cs` nginx"
RUN apt-get install -y nginx
RUN rm -rf /var/lib/apt/lists/
RUN echo "\ndaemon off;" >> /etc/nginx/nginx.conf
EXPOSE 80
CMD ["/usr/sbin/nginx"]

Answer 1

Docker image layers serve as a cache for subsequent builds. Without some sort of change in the Dockerfile, you're likely getting nginx 1.10.3 because it was cached from a previous build.
Instead of building your own nginx image, you should use the official nginx image, and choose the tag (e.g., 1.15.9) for the version you want.

Answer 2

First off, trivially, you need to apt-get update to fetch the index files from the repos you added before apt will find any packages there.
RUN add-apt-repository blah blah
RUN apt-get update -y # Add this
RUN apt-get install -y whatever
But also, you have invalid repos in the add-apt-repository section. The output of lsb_release -sc is a Debian code name like stretch which of course the Canonical partner repo doesn't have a section for; and the NGninx repo only supports Debian squeeze (though I would expect the packages to also work on newer versions of Debian).
Finally, you need to manage the keys of these repos, or otherwise mark them as safe. As a small bonus, I tried to condense your apt-get downloads slightly. Try this Dockerfile:
FROM debian:latest
RUN apt-get -y update
RUN apt-get install -yq gnupg2 \
software-properties-common curl # lsb-release
# XXX FIXME: the use of [trusted=yes] is really quick and dirty
RUN add-apt-repository "deb [trusted=yes] http://archive.canonical.com/ bionic partner"
RUN add-apt-repository "deb [trusted=yes] http://nginx.org/packages/debian squeeze nginx"
RUN apt-get update -y
RUN apt-get install -y nginx
RUN rm -rf /var/lib/apt/lists/
RUN echo "\ndaemon off;" >> /etc/nginx/nginx.conf
EXPOSE 80
CMD ["/usr/sbin/nginx"]