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What is Debian?
Debian is an operating system which is composed primarily of free and open-source software, most of which is under the GNU General Public License, and developed by a group of individuals known as the Debian project. Debian is one of the most popular Linux distributions for personal computers and network servers, and has been used as a base for several other Linux distributions.
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About this image
The images in this repository are intended to be as minimal as possible (because of the immutable/layered nature of containers, it's much easier to add than it is to remove). More specifically, they're built from the "minbase" variant, which only installs "required" packages, and thus creates the smallest possible footprint that is still "Debian" (as defined/managed by the Release and FTP teams within the project).
The %%IMAGE%%:latest tag will always point the latest stable release. Stable releases are also tagged with their version (ie, %%IMAGE%%:11 is an alias for %%IMAGE%%:bullseye, %%IMAGE%%:10 is an alias for %%IMAGE%%:buster, etc).
The rolling tags (%%IMAGE%%:stable, %%IMAGE%%:testing, etc) use the rolling suite names in their /etc/apt/sources.list file (ie, deb http://deb.debian.org/debian testing main).
The mirror of choice for these images is the deb.debian.org CDN pointer/redirector so that it's as reliable as possible for the largest subset of users (and is also the default mirror for debootstrap as of 2016-10-20). See the deb.debian.org homepage for more information.
If you find yourself needing a Debian release which is EOL (and thus only available from archive.debian.org), you should check out the debian/eol image, which includes tags for Debian releases as far back as Potato (Debian 2.2), the first release to fully utilize APT.
Locales
Given that it is a faithful "minbase" install of Debian, this image only includes the C, C.UTF-8, and POSIX locales by default. For most uses requiring a UTF-8 locale, C.UTF-8 is likely sufficient (-e LANG=C.UTF-8 or ENV LANG C.UTF-8).
For uses where that is not sufficient, other locales can be installed/generated via the locales package. PostgreSQL has a good example of doing so, copied below:
RUN apt-get update && apt-get install -y locales && rm -rf /var/lib/apt/lists/* \
&& localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8
ENV LANG en_US.utf8
How It's Made
The rootfs tarballs for this image are built using the reproducible-Debian-rootfs tool, debuerreotype, with an explicit goal being that they are transparent and reproducible. Using the same toolchain, it should be possible to regenerate (clean-room!) the same tarballs used for building the official Debian images. The examples/debian.sh script in that debuerreotype repository (and the debian-all.sh companion/wrapper) is the canonical entrypoint used for creating the artifacts published in this image (via a process similar to the docker-run.sh included in the root of that repository).
Additionally, the scripts in %%GITHUB-REPO%% are used to create each tag's Dockerfile and collect architecture-specific tarballs into dist-ARCH branches on the same repository, which also contain extra metadata about the artifacts included in each build, such as explicit package versions included in the base image (rootfs.manifest), the exact snapshot.debian.org timestamp used for debuerreotype invocation (rootfs.debuerreotype-epoch), the sources.list found in the image (rootfs.sources-list) and the one used during image creation (rootfs.sources-list-snapshot), etc.
For convenience, the SHA256 checksum (and full build command) for each of the primary rootfs.tar.xz artifacts are also published at docker.debian.net.