docker image creation using qcow2

This is the recommended workflow for creating your own Docker image for your application using a qcow2 image

Pre-requisite :

Install the following applications before running the below commands

yum install libguestfs libguestfs-tools
yum install qemu-kvm libvirt
systemctl start libvirtd

Create a Tar ball

You can convert your qcow2 images to a docker image by creating a tar ball.

Follow the below steps to create a tar ball using your qcow2 image:

You need to identify the device to which your root is mapped on the QCOW2 image

[root@azsystem mnt]$ guestfish add /apps/docker/azapp-test-ol7-20230126.qcow2 : run : list-filesystems
/dev/sda1: vfat
/dev/sda2: ext4
/dev/vg00/opt: ext4
/dev/vg00/root: ext4
/dev/vg00/swap: swap
/dev/vg00/tmp: ext4
/dev/vg00/var: ext4
/dev/vg00/var_log: ext4
/dev/vg00/var_log_audit: ext4
/dev/vg00/var_log_audit.d: ext4
/dev/vg00/var_tmp: ext4

Use /dev/vg00/root as it is the device mount that contains the root contents.

[root@azsystem mnt]$ mkdir /mnt/

[root@azsystem mnt]$ guestmount -a /apps/docker/azapp-test-ol7-20230126.qcow2 -m /dev/vg00/root /mnt/

[root@azsystem mnt]$ cd /mnt/

[root@azsystem mnt]$ ll /mnt
total 32
lrwxrwxrwx.   1 root root    7 Oct 12  2021 bin -> usr/bin
dr-xr-xr-x.   4 root root 4096 Dec 13  2021 boot
drwxr-xr-x.   2 root root    6 Oct 12  2021 dev
drwxr-xr-x. 103 root root 8192 Jan 11  2022 etc
drwxr-xr-x.   5 root root   46 Dec 13  2021 home
lrwxrwxrwx.   1 root root    7 Oct 12  2021 lib -> usr/lib
lrwxrwxrwx.   1 root root    9 Oct 12  2021 lib64 -> usr/lib64
drwxr-xr-x.   2 root root    6 Apr 10  2018 media
drwxr-xr-x.   2 root root    6 Dec 13  2021 mnt
drwx------.   2 root root    6 Jan 11  2022 oem
drwxr-xr-x.  10 root root 4096 Dec 13  2021 opt
drwxr-xr-x.   2 root root    6 Oct 12  2021 proc
dr-xr-x---.   8 root root 4096 Jan 11  2022 root
drwxr-xr-x.   2 root root    6 Oct 12  2021 run
lrwxrwxrwx.   1 root root    8 Oct 12  2021 sbin -> usr/sbin
drwxr-xr-x.   2 root root    6 Apr 10  2018 srv
drwxr-xr-x.   2 root root    6 Oct 12  2021 sys
drwxrwxrwt.   7 root root 4096 Jan 11  2022 tmp
drwxr-xr-x.  13 root root 4096 Oct 12  2021 usr
drwxr-xr-x.   3 root root   33 Jan 11  2022 var

[root@azsystem mnt]$ tar -czf /apps/docker/azapp-test-ol7-20230126-qcow2.tar.gz .

[root@azsystem mnt]$ ls -lh /apps/docker/azapp-test-ol7-20230126-qcow2.tar.gz
-rw------- 1 root root 1.5G Jan 25 16:23 /apps/docker/azapp-test-ol7-20230126-qcow2.tar.gz

Run Docker Import

Once you have your qcow2 tar ball created, ran the docker import command with "EXPOSE 22" to ensure you can SSH into the container once the image is created


[root@azsystem mnt]$ cat /apps/docker/azapp-test-ol7-20230126-qcow2.tar.gz | sudo docker import -c "EXPOSE 22" - azapp-test-ol7-20230126-qcow2:0.0.1
sha256:8bbcaa7a34e50893de1b804e48dc04733ad5ff7d91aba890644a8c12e2798070
[root@azsystem mnt]$ docker images
REPOSITORY                                     TAG                 IMAGE ID            CREATED             SIZE
azapp-test-ol7-20230126-qcow2                  0.0.1               8bbcaa7a34e5        24 seconds ago      4.08GB
azapp-test-ol7-20230126                        0.0.1               569ef6347563        4 hours ago         2.66GB
azapp-oel7-test-image                            0.0.2               5f60b5e9fbd0        13 days ago         3.24GB
azapp-oel6-test-image                            0.0.2               13dad8fe7b85        2 weeks ago         2.3GB

Docker Run

The docker image has been created successfully now you can use docker run command to get inside the container

[root@azsystem mnt]$ docker run -it  -v /git/:/home/rpmbuild/git -v /var/run/docker.sock:/var/run/docker.sock azapp-test-ol7-20230126-qcow2:0.0.1 bash

[root@c95aaf034a04 /]$ ls -ltr
total 4
drwxr-xr-x   2 root root    6 Apr 11  2018 srv
drwxr-xr-x   2 root root    6 Apr 11  2018 media
drwxr-xr-x   2 root root    6 Oct 13  2021 run
drwxr-xr-x  13 root root  155 Oct 13  2021 usr
lrwxrwxrwx   1 root root    8 Oct 13  2021 sbin -> usr/sbin
lrwxrwxrwx   1 root root    9 Oct 13  2021 lib64 -> usr/lib64
lrwxrwxrwx   1 root root    7 Oct 13  2021 lib -> usr/lib
lrwxrwxrwx   1 root root    7 Oct 13  2021 bin -> usr/bin
dr-xr-xr-x   4 root root 4096 Dec 13  2021 boot
drwxr-xr-x  10 root root  133 Dec 13  2021 opt
drwxr-xr-x   2 root root    6 Dec 13  2021 mnt
dr-xr-x---   1 root root   20 Jan 11  2022 root
drwx------   2 root root    6 Jan 11  2022 oem
drwxrwxrwt   7 root root  138 Jan 11  2022 tmp
drwxr-xr-x   1 root root   66 Jan 25 23:34 etc
dr-xr-xr-x 434 root root    0 Jan 25 23:34 proc
dr-xr-xr-x  13 root root    0 Jan 25 23:34 sys
drwxr-xr-x   1 root root   17 Jan 25 23:34 var
drwxr-xr-x   1 root root   22 Jan 25 23:34 home
drwxr-xr-x   5 root root  360 Jan 25 23:34 dev

Optimize your Image Size

Docker images can get very large very soon, and that will become a problem when pulling over the network or pushing on devices with limited storage. Here are a some suggestions on how to keep your image size small:

Reduce the number of RUN commands in your Docker. Each command adds a layer to the image, so consolidating the number of RUN can reduce the number of layers in the final image.

Note: Docker image layers are designed to be reusable, and will not be pushed or pulled if they've not changed.
Use --no-install-recommends when installing packages with apt-get install to disable installations of optional packages and save some disk space.
Remove tar balls or other archive files that were copied during the installation. Each layer is added on top of the others, so files that were not removed in a given RUN step will be present in the final image even if they are removed in a later RUN step.
Also, clean up your package lists that are downloaded with apt-get update by removing /var/lib/apt/lists/* in the same RUN step.
Create separate images for development and production. Production images should not include all of the libraries and dependencies pulled in by the build.
Use multi-stage builds (see Docker docs) and push only your prod image.

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