[peterwwillis]

> Every time you run a container from that image built from a Dockerfile, you'll get the same filesystem & environment

The image (and thus container) are the same. Trying to rebuild it from scratch leaves it not the same, typically because the way people put together Dockerfiles and build images does not follow a standard. And it has to do with the container culture.

Let's take a Dockerfile for the CentOS 7 version of nginx (https://github.com/CentOS/CentOS-Dockerfiles/blob/master/ngi...)

  FROM centos:centos7
  MAINTAINER The CentOS Project <[e-mail]>
  
  RUN yum -y update; yum clean all

Right off the bat I think: what the hell? Why are they doing a yum update? A yum update today may very well leave the system in a completely different state than a year ago. There's likely been some package updated in that time, which changes the state of the system. Right off the bat we're screwed.

  RUN yum -y install epel-release tar ; yum clean all
  RUN yum -y install nginx ; yum clean all

And what the hell is this?! There's no version, no build, no checksum. What the hell did we just install? If those packages change in a year, we're screwed. Not to mention 'epel-release' and 'tar' should be set as dependencies somewhere, and the type of dependencies too.

  ADD nginx.conf /etc/nginx/nginx.conf
  RUN echo "daemon off;" >> /etc/nginx/nginx.conf

Oh, cool, just use whatever the hell this config is which may or may not be different from the one that shipped with the original package. And let's just modify it for no apparent reason, too. And definitely make sure we have no way to know what version of that file we're using for this image. Lovely.

  RUN curl https://git.centos.org/sources/httpd/c7/acf5cccf4afaecf3afeb18c50ae59fd5c6504910 \
      | tar -xz -C /usr/share/nginx/html \
      --strip-components=1

The hell? We're pulling some random git sources from a git server which i'm willing to bet has no standard mirrors? On closer inspection it doesn't really look like a git server, but a host named git which hosts files whose names are a checksum, though we don't know where this is from or what exactly it refers to. If this server or file disappears, good luck knowing what in hell was being downloaded here.

  RUN sed -i -e 's/Apache/nginx/g' -e '/apache_pb.gif/d' \ 
      /usr/share/nginx/html/index.html
  
  EXPOSE 80
  
  CMD [ "/usr/sbin/nginx" ]