A "Hello Stacker" Image¶
Stacker builds OCI images using the YAML definition in a file referred to as the Stacker file. Let's create a stacker file definition that packages a simple "Hello Stacker!" script into an OCI container image.
hello-stacker:
from:
type: docker
url: docker://zothub.io/tools/busybox:stable
run: |
mkdir -p /hello-stacker-app
echo 'echo "Hello Stacker!"' > /hello-stacker-app/hello.sh
chmod +x /hello-stacker-app/hello.sh
entrypoint: /hello-stacker-app/hello.sh
hello-stacker, the first key in the above YAML, builds an OCI image named hello-stacker.
The from section defines that hello-stacker image builds on an existing image called zothub.io/tools/busybox:stable, and the base image is of type docker downloaded from URL docker://zothub.io/tools/busybox:stable.
The run section defines a build script. This script is executed on the ubuntu image's root file system and creates an executable file called /hello-stacker-app/hello.sh.
entrypoint defines that /hello-stacker-app/hello.sh is executed as the init process on creating this container by a container runtime.
We can create the hello-stacker image with this input as a stacker file name hello_stacker.yaml.
$ stacker build -f hello_stacker.yaml
preparing image hello-stacker...
loading docker://zothub.io/tools/busybox:stable
Copying blob cf92e523b49e done
Copying config cb52c703ef done
Writing manifest to image destination
Storing signatures
+ mkdir -p /hello-stacker-app
+ echo echo "Hello Stacker!"
+ chmod +x /hello-stacker-app/hello.sh
filesystem hello-stacker built successfully
hello-stacker.yaml as input, downloaded the zothub.io/tools/busybox:stable image from docker hub and generated an OCI image with tag hello-stacker. We can verify this using stacker inspect: $ stacker inspect
hello-stacker
layer 0: cf92e523b49e... (30 MB, application/vnd.oci.image.layer.v1.tar+gzip)
layer 1: 2910d371807c... (176 B, application/vnd.oci.image.layer.v1.tar+gzip)
Annotations:
io.stackeroci.stacker.stacker_yaml: hello-stacker:
from:
type: docker
url: docker://zothub.io/tools/busybox:stable
run: |
mkdir -p /hello-stacker-app
echo 'echo "Hello Stacker!"' > /hello-stacker-app/hello.sh
chmod +x /hello-stacker-app/hello.sh
entrypoint: /hello-stacker-app/hello.sh
Image config:
{
"created": "2022-10-24T03:47:24.374578534Z",
"author": "stacker-dev",
"architecture": "amd64",
"os": "linux",
"config": {
"Env": [
"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
],
"Entrypoint": [
"/hello-stacker-app/hello.sh"
]
},
"rootfs": {
"type": "layers",
"diff_ids": [
"sha256:17f623af01e277c5ffe6779af8164907de02d9af7a0e161662fc735dd64f117b",
"sha256:a33ac624796108439088267fb525f1038dccb6f6a50796a1593786a31d97a4bd"
]
},
"history": [
{
"created": "2022-10-04T23:35:20.465021967Z",
"created_by": "/bin/sh -c #(nop) ADD file:6cd2e13356aa5339c1f2abd3c210a52f6ed74fae05cd61aa09f37b6a4764f65c in / "
},
{
"created": "2022-10-04T23:35:20.857335994Z",
"created_by": "/bin/sh -c #(nop) CMD [\"bash\"]",
"empty_layer": true
},
{
"created": "2022-10-24T03:47:24.372946511Z",
"created_by": "stacker build of hello-stacker"
},
{
"created": "2022-10-24T03:47:24.374578534Z",
"created_by": "stacker build",
"author": "root@ins15-pp24-ru11",
"empty_layer": true
}
]
}
Note that if we rebuild the image without any modifications to the stacker file, less things happen:
$ stacker build -f hello_stacker.yaml
preparing image hello-stacker...
loading docker://zothub.io/tools/busybox:stable
Copying blob cf92e523b49e skipped: already exists
Copying config cb52c703ef done
Writing manifest to image destination
Storing signatures
found cached layer hello-stacker
Stacker will cache all inputs to the stacker file and rebuilds the image only if something changed in the stacker file. The cache and the metadata required to track the build state live in the .stacker directory where the stacker command is run. Stacker cache can be cleaned using stacker clean command.