A container image is a shareable application artifact that bundles an application with its libraries, executables, configuration, and runtime requirements. It isolates this software from the host while normally sharing only the host kernel, making development, testing, and deployment more consistent. section 1.2.1 section 2.2.1
An OCI container image contains:
- A root filesystem directory tree containing the software required by the application. section 1.2.3
- JSON configuration describing the rootfs, executable, working directory, environment variables, maintainer, and identifying labels. section 1.2.3
- A manifest list linking architecture-specific images so the same image name can resolve on different machines. section 1.2.3
- Tar-packed layers and JSON files stored locally or in transports such as registries, OCI archives, and Docker archives. section 1.2.3 section 2.2.1
An image is not a running container. Podman pulls image layers into local container storage; then it combines image configuration, engine defaults, and user input into an OCI runtime specification. The runtime applies namespaces, cgroups, and security constraints before launching the container process. section 1.2.3 section 2.2.1
Pull, inspect, and run an image
Use `podman run -ti --rm registry.access.redhat.com/ubi8/httpd-24 bash` to pull an image when absent locally and open an interactive shell. `podman inspect` exposes configuration such as environment variables and the default command. Running the same image again reuses locally stored layers; omitting `bash` runs the image's default application, Apache HTTP Server. section 2.1.1
Build and share an image
Build images with `podman build` from a Containerfile or Dockerfile, then publish them with `podman push`. A manually committed image can be created with `podman commit`, but scripted builds are the usual approach because they make image creation repeatable and help maintain current, secure contents. section 2.3 section 2.2.4
$ podman image tree myimage Image ID: 2c7e43d88038 Tags: [localhost/myimage:latest] Size: 461.7MB Image Layers ├── ID: e39c3abf0df9 Size: 233.6MB ├── ID: 42c81bd2b468 Size: 20.48kB Top Layer of: [registry.access.redhat.com/ubi8:latest] ├── ID: 51a7beaa0b88 Size: 57.43MB ├── ID: 519e681b5702 Size: 170.6MB Top Layer of: [registry.access.redhat.com/ubi8/httpd-24:latest] └── ID: bc3dcdefdac3 Size: 69.63kB Top Layer of: [localhost/myimage:latest localhost/myapp:latest]
Inspect an image layer tree to see how base images and later application changes combine into a reusable artifact. section 2.2.1
