Add readme.

README.adoc

@@ -0,0 +1,43 @@
+= Docker images
+
+My amateurish attempts at creating docker images.
+
+== tastytea/gentoo-base
+
+A minimal Gentoo image without build dependencies. I use it as a base for other
+images, so that I don't have to compile everything from scratch all the time.
+
+=== Features
+
+* Replaced `-O2` with `-Os` in `${COMMON_FLAGS}`.
+* Turned off sandboxing (https://bugs.gentoo.org/680456[Bug 680456]).
+* `MAKEOPTS="-j1"`.
+* Deactivated some use-flags, added `minimal`.
+* `QUICKPKG_DEFAULT_OPTS="--include-config=y"`.
+* Packages installed: `sys-apps/portage` + run-time dependencies.
+* Deleted: Documentation, translations, time zone info.
+
+=== Example Dockerfile
+
+[source,docker]
+--------------------------------------------------------------------------------
+# Portage tree.
+FROM gentoo/portage:latest as portage
+
+# Minimal Gentoo image without build dependencies.
+FROM tastytea/gentoo-base:latest as gentoo-base
+
+# Build packages in /workdir.
+FROM gentoo/stage3-amd64:latest as build
+COPY --from=portage /var/db/repos/gentoo /var/db/repos/gentoo
+COPY --from=gentoo-base / /workdir
+RUN cp /workdir/etc/portage/make.conf /etc/portage/make.conf
+RUN eselect profile set default/linux/amd64/17.1
+RUN echo 'games-misc/cowsay' >> \
+    /workdir/etc/portage/package.accept_keywords/cowsay
+RUN emerge --root=/workdir --quiet games-misc/cowsay
+
+# Final image.
+FROM scratch
+COPY --from=build /workdir /
+--------------------------------------------------------------------------------