Exercise: Build and version an image

Build a small web image from a Containerfile, read a build error, run the image, make a second version, and move a tag between versions.

Build and version an image

Write a Containerfile and build a tagged image.
Read a failed COPY step.
Run and compare two versions of an image.

Start the exercise

On workstation, start the exercise. It installs podman on servera and pulls the base image httpd-24 for student.

[student@workstation ~]$ lab start sa-containers-build

Later tasks assume that you are logged in to the server they name: ssh student@servera from workstation, then sudo -i for a root shell.

The project directory

As student on servera, create ~/portal with index.html containing Portal is up. Make sure the base image registry.access.redhat.com/ubi9/httpd-24:latest is pulled.

[student@servera ~]$ podman pull registry.access.redhat.com/ubi9/httpd-24:latest | tail -1
4e74cc90c2a4ab4fcc97976b9e382cd2b8e0711222a30206ae8089fc39b01bb9
[student@servera ~]$ mkdir -p ~/portal && cd ~/portal
[student@servera portal]$ echo "Portal is up" > index.html

A Containerfile with a mistake

Write a Containerfile that starts from the base image and copies missing.html to /var/www/html/index.html. Build it as localhost/portal:bad and read the error.

[student@servera portal]$ printf 'FROM registry.access.redhat.com/ubi9/httpd-24:latest\nCOPY missing.html /var/www/html/index.html\n' > Containerfile
[student@servera portal]$ podman build -t localhost/portal:bad . 2>&1 | tail -2
STEP 2/2: COPY missing.html /var/www/html/index.html
Error: building at STEP "COPY missing.html /var/www/html/index.html": checking on sources under "/home/student/portal": copier: stat: "/missing.html": no such file or directory

The build stopped at step 2 because that file is not in the build context (the current directory).

Fix it and build version 1

Correct the Containerfile to copy index.html, and build localhost/portal:1. List the images.

[student@servera portal]$ printf 'FROM registry.access.redhat.com/ubi9/httpd-24:latest\nCOPY index.html /var/www/html/index.html\n' > Containerfile
[student@servera portal]$ podman build -t localhost/portal:1 . | tail -2
Successfully tagged localhost/portal:1
8783f7652eb1f58df91e9c935a51986cbb9e3de30809eb517a08e5e4f883b136
[student@servera portal]$ podman images --format "{{.Repository}}:{{.Tag}} {{.ID}}"
localhost/portal:1 8783f7652eb1
registry.access.redhat.com/ubi9/httpd-24:latest 4e74cc90c2a4

Run it

Run localhost/portal:1 as portal, publishing host port 8081 to the container's 8080. Wait two seconds and fetch the page.

[student@servera portal]$ podman run -d --name portal -p 8081:8080 localhost/portal:1
[student@servera portal]$ sleep 2; curl -s localhost:8081
Portal is up

No volume was needed: the page is part of the image.

Version 2 and a moving tag

Change the page to Portal v2, build localhost/portal:2 and also tag it localhost/portal:latest. List the images: which two share an image ID? Replace the running container with one from latest and fetch the page again.

[student@servera portal]$ echo "Portal v2" > index.html
[student@servera portal]$ podman build -t localhost/portal:2 . | tail -1
65755b3c59904919657cad23be6f3e8e87d42d648de4c04cb3cc43b15efc1f88
[student@servera portal]$ podman tag localhost/portal:2 localhost/portal:latest
[student@servera portal]$ podman images --format "{{.Repository}}:{{.Tag}} {{.ID}}" | grep portal
localhost/portal:latest 65755b3c5990
localhost/portal:2 65755b3c5990
localhost/portal:1 8783f7652eb1
[student@servera portal]$ podman rm -f -t 1 portal
portal
[student@servera portal]$ podman run -d --name portal -p 8081:8080 localhost/portal:latest
[student@servera portal]$ sleep 2; curl -s localhost:8081
Portal v2

latest and 2 are two names for the same image. Move latest when you release; a deployment that names :1 keeps getting the old page.

Look at the layers and the digest

Show the history of localhost/portal:2 (the first three lines) and the digest of the base image.

[student@servera portal]$ podman history localhost/portal:2 | head -3 | cut -c1-90
ID            CREATED                 CREATED BY                                     SIZE
cf1e25990315  Less than a second ago  /bin/sh -c #(nop) COPY file:702c4212… in /var/www/html/index.html  2.56kB
[student@servera portal]$ podman image inspect registry.access.redhat.com/ubi9/httpd-24 --format '{{.Digest}}'
sha256:ea005876daf0c5ee8024c9334503c742a37ed5391ee0e32a9051540425298d76

For a build that must be repeatable, write the base as FROM registry.access.redhat.com/ubi9/httpd-24@sha256:….

Grade and finish

Check your work with lab grade sa-containers-build, then put the exercise away with lab finish sa-containers-build.