apt-get update in a dockerfile destroys app consistency? - dockerfile

Question

In the dockerfiles I have seen, and the in the best practices for writing a docker file: https://docs.docker.com/engine/reference/builder/#copy, when apt-get is used to install some packages, apt-get update is always run first. I have a concern on this because the app we build in the corresponding docker container would depend on these installed packages, if there is some inconsistency in the newest version of the installed packages, the software we build will not work right any more. Why we do not specify a version of the packages, but use apt-get update instead?

Answer 1

From the man page for apt-get:
update is used to resynchronize the package index files from their
sources. The indexes
of available packages are fetched from the location(s) specified in
/etc/apt/sources.list. For example, when using a Debian archive, this command retrieves
and scans the Packages.gz files, so that information about new and updated packages is
available. An update should always be performed before an upgrade or dist-upgrade.
Please be aware that the overall progress meter will be incorrect as the size of the
package files cannot be known in advance.
You can try running apt-get install without running update on a docker image but you'll probably find that a lot of things will fail to install because the package indexes are out of date.
Once you update the package data, then you can specify a specific version for packages when you run install e.g.
apt update && apt install -y \
git=1:2.7.4-0ubuntu1.4
Example with docker container:
> sudo docker run -it ubuntu:16.04 /bin/bash
# root#513eb786d86d:/# apt install git
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package git
root#513eb786d86d:/# apt install git=1:2.7.4-0ubuntu1.4
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package git
root#513eb786d86d:/# apt update
...
root#513eb786d86d:/# apt install git=1:2.7.4-0ubuntu1.4
# works this time!