A developer has created a new image to use in a Docker build and has added a tag for the image by using the command: $ docker tag 84fe411926287 local/app:0.4 Which command must be executed next to build the Docker image using the tag?

The docker build command is used to create a Docker image from a Dockerfile and a context. The -t (or --tag) flag is the correct option to apply a name and an optional tag to the image being built. The command in option D correctly uses docker build with the -t flag to assign the tag local/app:0.4 to the new image.

While the question's premise is slightly flawed (the docker tag command is used on an existing image, whereas docker build -t is used when creating a new one), the core task is to identify the command that builds an image and applies a tag, which is precisely what docker build -t does.

A. The -p flag is invalid for the docker build command. It is used with docker run to publish a container's ports to the host.

B. The docker run command is used to create and start a container from an existing image, not to build a new image.

C. The docker run command is used to create and start a container from an existing image, not to build a new image.

1. Docker Official Documentation, docker build command reference: "The -t, --tag flag... Name and optionally a tag in the 'name:tag' format." This source confirms that -t is the correct flag for tagging an image during the build process.

Source: Docker Inc., Docker Docs, "docker build," https://docs.docker.com/engine/reference/commandline/build/#tag

2. Docker Official Documentation, docker run command reference: This document details the valid options for the docker run command, including -p for port mapping and -t for allocating a pseudo-TTY. It confirms these flags are not part of the docker build command.

Source: Docker Inc., Docker Docs, "docker run," https://docs.docker.com/engine/reference/commandline/run/#options

3. Docker Official Documentation, docker tag command reference: "Creates a tag TARGETIMAGE that refers to SOURCEIMAGE." This clarifies that docker tag operates on an already existing image, which highlights the confusing premise in the question but does not change the correct answer for building with a tag.

Source: Docker Inc., Docker Docs, "docker tag," https://docs.docker.com/engine/reference/commandline/tag/