Explanation
The FROM statement in a Dockerfile specifies the base image from which the subsequent instructions are executed1. The value of the FROM statement can be either an image name, an image name with a tag, or an image ID1. The image name can be either a repository name or a repository name with a registry prefix2. For example, ubuntu is a repository name, and docker.io/ubuntu is a repository name with a registry prefix2. The tag is an optional identifier that can be used to specify a particular version or variant of an image1. For example, ubuntu:focal refers to the image with the focal tag in the ubuntu repository2. The image ID is a unique identifier that is automatically generated when an image is built or pulled1. For example, sha256:9b0dafaadb1cd1d14e4db51bd0f4c0d56b6b551b2982b2b7c637ca143ad605d2 is an image ID3.
Therefore, the only valid value in the FROM statement among the given options is ubuntu:focal, which is an image name with a tag. The other options are invalid because:
* docker://ubuntu:focal is not a valid image name format. The docker:// prefix is used to specify a transport protocol, not a registry prefix4.
* registry:ubuntu:focal is not a valid image name format. The registry prefix should be a valid hostname or IP address, not a generic term2.
* file:/tmp/ubuntu/Dockerfile is not a valid image name format. The file: prefix is used to specify a local file path, not an image name5.
* http://docker.example.com/images/ubuntu-focal.iso
is not a valid image name format.
The
http:// prefix is used to specify a web URL, not an image name
5.
References:
* 1: Dockerfile reference | Docker Docs
* 2: docker - Using FROM statement in dockerfile - Stack Overflow
* 3: How to get the image id from a docker image - Stack Overflow
* 4: skopeo - Docker Registry v2 API tool - Linux Man Pages (1)
* 5: How to build a Docker image from a local Dockerfile? - Stack Overflow