Docker docs and ACI docs decsribe the steps how to create base images from existing tarballs/folders with root file systems of distribution. If you make a deeper look, you will probably find the CentOS tarballs which are used by docker for creation of centos base images.
But how to get this root file system tree? This blogpost covers the creation of this root file system tree for CentOS and the creation of base images for Docker and Rkt.
CentOS root file system tree
You need existing CentOS 7 system for this. You can use a vagrant environment if you want. Besides that, you have to exectute the steps below as root.
# Create a folder for our new root structure
$ export centos_root='/centos_image/rootfs'
$ mkdir -p $centos_root
# initialize rpm database
$ rpm --root $centos_root --initdb
# download and install the centos-release package, it contains our repository sources
$ yum reinstall --downloadonly --downloaddir . centos-release
$ rpm --root $centos_root -ivh --nodeps centos-release*.rpm
$ rpm --root $centos_root --import $centos_root/etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
# install yum without docs and install only the english language files during the process
$ yum -y --installroot=$centos_root --setopt=tsflags='nodocs' --setopt=override_install_langs=en_US.utf8 install yum
# configure yum to avoid installing of docs and other language files than english generally
$ sed -i "/distroverpkg=centos-release/a override_install_langs=en_US.utf8\ntsflags=nodocs" $centos_root/etc/yum.conf
# chroot to the environment and install some additional tools
$ cp /etc/resolv.conf $centos_root/etc
# mount the device tree, as its required by some programms
$ mount -o bind /dev $centos_root/dev
$ chroot $centos_root /bin/bash <<EOF
yum install -y procps-ng iputils
yum clean all
EOF
$ rm -f $centos_root/etc/resolv.conf
$ umount $centos_root/devDocker base image
# install and enable docker
$ yum install -y docker
...
$ systemctl start docker
# create docker image
$ tar -C $centos_root -c . | docker import - centos
sha256:28843acfa85226385d2db0196db72465729db38088ea98751d4a5c3d4c85ccd
$ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
centos latest 28843acfa852 11 seconds ago 287.7 MB
# run docker image
$ docker run --rm centos cat /etc/redhat-release
CentOS Linux release 7.2.1511 (Core)Via usual docker ways you can now export the image or push it to some registry.
ACI image for rkt
You will nedd actool to build an ACI image from root tree. You can build it by yourself or get it from my OBS repository:
$ wget http://download.opensuse.org/repositories/home:artem_sidorenko:rkt/CentOS_7/home:artem_sidorenko:rkt.repo \
-O /etc/yum.repos.d/rkt.repo
$ yum install -y actool
...
# its a good idea to install rkt from same repository to verify the image
$ yum install -y rkt
...Rkt uses ACI images and they require a manifest file, which describes the image.
$ export centos_manifest='/centos_image/manifest'
$ cat > $centos_manifest <<EOF
{
"acKind": "ImageManifest",
"acVersion": "0.0.1",
"name": "centos",
"labels": [
{"name": "os", "value": "linux"},
{"name": "arch", "value": "amd64"},
{"name": "version", "value": "0.0.1"}
],
"app": {
"exec": [
"/bin/bash"
],
"user": "0",
"group": "0"
}
}
EOF
$ actool build /centos_image centos.aci
# import the image to rkt
$ rkt fetch --insecure-options=image centos.aci
$ rkt image list
ID NAME IMPORT TIME LAST USED SIZE LATEST
sha512-a8d4915265a0 centos 16 seconds ago 15 seconds ago 558MiB false
# run the image with rkt
$ rkt run centos --exec cat -- /etc/redhat-release ---
image: using image from local store for image name coreos.com/rkt/stage1-coreos:1.9.1
image: using image from local store for image name centos
networking: loading networks from /etc/rkt/net.d
networking: loading network default with type ptp
[ 4135.403735] centos[5]: CentOS Linux release 7.2.1511 (Core)