apt-get clean and other strategies for keeping image size small?

[cboettig]

Should we be using apt-get clean in the RUN lines that call apt-get update && apt-get install to avoid unnecessary image space? not sure how large those files are.

[eddelbuettel]

At some point I started to have the suspicion that Docker does that -- as I never see deb files in /var/cache/apt/archives. But maybe I tricked myself ...

[cboettig]

Yeah, I'm confused because on debian:testing if I try apt-get install ed without running apt-get update it just says package not found, but on r-base it runs without an update:

 docker run --rm -it rocker/r-base bash
root@8c2f850457d6:/# apt-get install ed
Reading package lists... Done
Building dependency tree       
Reading state information... Done
ed is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.

Seems like it has found some package lists to read? But then running apt-get clean doesn't change this behavior, so I'm confused about what package list it's reading...

[wch]

It does automatically do a clean after each package install, which you can see with this command:

$ docker run --rm debian cat /etc/apt/apt.conf.d/docker-clean

# Since for most Docker users, package installs happen in "docker build" steps,
# they essentially become individual layers due to the way Docker handles
# layering, especially using CoW filesystems.  What this means for us is that
# the caches that APT keeps end up just wasting space in those layers, making
# our layers unnecessarily large (especially since we'll normally never use
# these caches again and will instead just "docker build" again and make a brand
# new image).

# Ideally, these would just be invoking "apt-get clean", but in our testing,
# that ended up being cyclic and we got stuck on APT's lock, so we get this fun
# creation that's essentially just "apt-get clean".
DPkg::Post-Invoke { "rm -f /var/cache/apt/archives/*.deb /var/cache/apt/archives/partial/*.deb /var/cache/apt/*.bin || true"; };
APT::Update::Post-Invoke { "rm -f /var/cache/apt/archives/*.deb /var/cache/apt/archives/partial/*.deb /var/cache/apt/*.bin || true"; };

Dir::Cache::pkgcache "";
Dir::Cache::srcpkgcache 

# Note that we do realize this isn't the ideal way to do this, and are always
# open to better suggestions (https://github.com/docker/docker/issues).

Edit: there are a couple other files in that directory that may be relevant (languages, etc).

[eddelbuettel]

@cboettig I fear you are getting confused between package downloads and metadata about packages.

@wch Thanks, I suspected something like that. Nice find.

[cboettig]

Thanks both!