Update from Jan 2015: The Zulu team added formal Debian support last October, I just did not know about it. Look at the version history for Zulu 8.4, 7.7, and 6.6 at http://www.azulsystems.com/zulurelnotes. Also look on DockerHub for their 8.4.x Docker files. They don't use lsb_release -cs in Debian Dockerfiles anymore, and instead allow the Zulu repository to honor 'stable' as release name. 'stable' always pushes the highest level for a Java major version. - I am paraphrasing the comments from Matthew Schuetze below.

I saw the following line in a Dockerfile

RUN echo "deb http://repos.azulsystems.com/ubuntu `lsb_release -cs` main" >> /etc/apt/sources.list.d/zulu.list

The lsb_release program is not part of the wheezy standard programs. But we can install it:

$ apt-get update && apt-get install -y lsb

How many files were created by that install?

$ docker diff 09 | wc -l
30013

Over 30,000 files!

I next tried being a bit more specific with

$ apt-get update && apt-get install -y lsb-release

How many files were created by that install?

$ docker diff 23 | wc -l
1689

I conclude that hard-coding "wheezy" is better than using lsb_release in a Dockerfile. At least when using Debian as the base operating system.

While I dabble in System Administration, I don't have a deep knowledge how packages are created or maintained. Today, we'll see how to use Docker to increase my understanding of "apt-get update". I was curious about this command because I read that it's good practice to remove the files created during the update process.

I started a small container using

docker run -i -t debian:wheezy /bin/bash

In another window, I found the ID of the running container using "docker ps". Let's pretend that ID starts with "45...". Look for any changed files using

docker diff "45"

You'll see nothing displayed. Now run "apt-get update" in the wheezy container. Then run the diff command again. You should see the following differences:

C /var
C /var/lib
C /var/lib/apt
C /var/lib/apt/lists
A /var/lib/apt/lists/http.debian.net_debian_dists_wheezy-updates_Release
A /var/lib/apt/lists/http.debian.net_debian_dists_wheezy-updates_Release.gpg
A /var/lib/apt/lists/http.debian.net_debian_dists_wheezy-updates_main_binary-amd64_Packages.gz
A /var/lib/apt/lists/http.debian.net_debian_dists_wheezy_Release
A /var/lib/apt/lists/http.debian.net_debian_dists_wheezy_Release.gpg
A /var/lib/apt/lists/http.debian.net_debian_dists_wheezy_main_binary-amd64_Packages.gz
A /var/lib/apt/lists/lock
C /var/lib/apt/lists/partial
A /var/lib/apt/lists/security.debian.org_dists_wheezy_updates_Release
A /var/lib/apt/lists/security.debian.org_dists_wheezy_updates_Release.gpg
A /var/lib/apt/lists/security.debian.org_dists_wheezy_updates_main_binary-amd64_Packages.gz

Inside the wheezy container we now know where to look to find file sizes:

# ls -lh /var/lib/apt/lists
total 8.0M
-rw-r--r-- 1 root root 121K Nov 23 02:49 http.debian.net_debian_dists_wheezy-updates_Release
-rw-r--r-- 1 root root  836 Nov 23 02:49 http.debian.net_debian_dists_wheezy-updates_Release.gpg
-rw-r--r-- 1 root root    0 Nov 23 02:37 http.debian.net_debian_dists_wheezy-updates_main_binary-amd64_Packages
-rw-r--r-- 1 root root 165K Oct 18 10:33 http.debian.net_debian_dists_wheezy_Release
-rw-r--r-- 1 root root 1.7K Oct 18 10:44 http.debian.net_debian_dists_wheezy_Release.gpg
-rw-r--r-- 1 root root 7.3M Oct 18 10:07 http.debian.net_debian_dists_wheezy_main_binary-amd64_Packages.gz
-rw-r----- 1 root root    0 Nov 23 04:09 lock
drwxr-xr-x 2 root root 4.0K Nov 23 04:09 partial
-rw-r--r-- 1 root root 100K Nov 20 16:31 security.debian.org_dists_wheezy_updates_Release
-rw-r--r-- 1 root root  836 Nov 20 16:31 security.debian.org_dists_wheezy_updates_Release.gpg
-rw-r--r-- 1 root root 270K Nov 20 16:31 security.debian.org_dists_wheezy_updates_main_binary-amd64_Packages.gz

Obviously, those .gz files might be interesting. It's easy enough to uncompress them:

gzip -d http.debian.net_debian_dists_wheezy_main_binary-amd64_Packages.gz

And now it's possible to see what's inside:

# more http.debian.net_debian_dists_wheezy_main_binary-amd64_Packages
Package: 0ad
Version: 0~r11863-2
Installed-Size: 8260
Maintainer: Debian Games Team 
Architecture: amd64
Depends: 0ad-data (>= 0~r11863), 0ad-data (<= 0~r11863-2), gamin | fam, libboost-signals1.49.0 (>= 1.49.0-1), libc6 (>= 2.11), libcurl3-gnutls (>= 7.16.2), libenet1a, libgamin0 | libfam0, libgcc1 (>= 1:4.1.1), libgl1-mesa-glx | libgl1, lib
jpeg8 (>= 8c), libmozjs185-1.0 (>= 1.8.5-1.0.0+dfsg), libnvtt2, libopenal1, libpng12-0 (>= 1.2.13-4), libsdl1.2debian (>= 1.2.11), libstdc++6 (>= 4.6), libvorbisfile3 (>= 1.1.2), libwxbase2.8-0 (>= 2.8.12.1), libwxgtk2.8-0 (>= 2.8.12.1), l
ibx11-6, libxcursor1 (>> 1.1.2), libxml2 (>= 2.7.4), zlib1g (>= 1:1.2.0)
Pre-Depends: dpkg (>= 1.15.6~)
Description: Real-time strategy game of ancient warfare
Homepage: http://www.wildfiregames.com/0ad/
Description-md5: d943033bedada21853d2ae54a2578a7b
Tag: game::strategy, implemented-in::c++, interface::x11, role::program,
 uitoolkit::sdl, uitoolkit::wxwidgets, use::gameplaying,
 x11::application
Section: games
Priority: optional
Filename: pool/main/0/0ad/0ad_0~r11863-2_amd64.deb
Size: 2260694
MD5sum: cf71a0098c502ec1933dea41610a79eb
SHA1: aa4a1fdc36498f230b9e38ae0116b23be4f6249e
SHA256: e28066103ecc6996e7a0285646cd2eff59288077d7cc0d22ca3489d28d215c0a

...

Given the information about the text file, we can find out how many packages are available:

# grep "Package" http.debian.net_debian_dists_wheezy_main_binary-amd64_Packages | wc -l
36237

Now you know why it's important to run the following in your Dockerfile after using apt-get to install software.

rm -rf /var/lib/apt/lists/*

Have fun exploring!