How to use the command 'docker build' (with examples)

The docker build command is used to build an image from a Dockerfile. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. This command is essential in the process of creating custom Docker images for specific applications or environments.

Use case 1: Build a docker image using the Dockerfile in the current directory

Code:

docker build .

Motivation: This use case is handy when you have a Dockerfile in the current directory and want to build an image based on it. By using the . as the build context, Docker will use all the files in the current directory to build the image.

Explanation: The . in the command specifies the build context, which tells Docker where to find the files and directories needed for building the image. By default, Docker looks for a file named ‘Dockerfile’ in the specified build context. In this case, the Dockerfile in the current directory will be used to build the image.

Example output: The Docker daemon will analyze the Dockerfile and start building the image, showing the progress and the steps being executed.

Use case 2: Build a docker image from a Dockerfile at a specified URL

Code:

docker build github.com/creack/docker-firefox

Motivation: This use case is helpful when you want to build an image from a Dockerfile hosted at a specific URL. It allows you to reuse Dockerfiles created by others without cloning their entire repositories.

Explanation: Instead of using a local file, you can provide a URL as the build context. In this case, Docker will fetch the Dockerfile from the specified URL and use it to build the image.

Example output: Docker will initiate a HTTP request to the given URL and retrieve the Dockerfile. It will then begin building the image using the fetched Dockerfile.

Use case 3: Build a docker image and tag it

Code:

docker build --tag name:tag .

Motivation: Tagging an image makes it easier to identify and manage versions of the image. It is helpful when you have different versions or variants of the same application that you want to differentiate.

Explanation: The --tag or -t flag allows you to add a custom tag to the image being built. In this case, name represents the name of the image, and tag represents the version or variant of the image.

Example output: After building the image, Docker will tag it with the provided name and tag. This can be seen in the list of local images by running docker images.

Use case 4: Build a docker image with no build context

Code:

docker build --tag name:tag - < Dockerfile

Motivation: Using a pipeline to provide the Dockerfile stream instead of a build context can be useful when you want to build an image directly from a Dockerfile without relying on the local file system.

Explanation: The - character in place of the build context indicates that the Dockerfile contents will be passed through the standard input rather than being read from a file on disk. The < operator is used to redirect the contents of the ‘Dockerfile’ file to the standard input.

Example output: The Docker daemon will read the Dockerfile contents from the standard input and use them to build the image, just like in the previous use cases.

Use case 5: Do not use the cache when building the image

Code:

docker build --no-cache --tag name:tag .

Motivation: Sometimes, you may want to ensure that the image is built from scratch, without using any pre-existing cached layers. This can be useful to verify that all dependencies and resources are up to date.

Explanation: The --no-cache flag tells Docker to ignore any cached layers and rebuild the image from scratch. This ensures that all the steps in the Dockerfile are executed, even if they were previously built and cached.

Example output: Docker will rebuild the image, ignoring the cache, and show the progress and steps being executed during the build process.

Use case 6: Build a docker image using a specific Dockerfile

Code:

docker build --file Dockerfile .

Motivation: In some scenarios, you may have multiple Dockerfiles in the build context, and you want to specify which one to use explicitly. This use case allows you to select a specific Dockerfile for building the image.

Explanation: The --file or -f flag is used to specify the path to the Dockerfile you want to use for building the image. In this case, Dockerfile represents the file name.

Example output: Docker will use the specified Dockerfile to build the image, showing the progress and steps being executed during the build process.

Use case 7: Build with custom build-time variables

Code:

docker build --build-arg HTTP_PROXY=http://10.20.30.2:1234 --build-arg FTP_PROXY=http://40.50.60.5:4567 .

Motivation: There might be a need to pass build-time variables to the Docker build process, which can be used to customize certain aspects of the image based on the environment or specific requirements.

Explanation: The --build-arg flag is used to pass build-time variables to the Docker build process. In this case, two variables, HTTP_PROXY and FTP_PROXY, are being set with their respective values.

Example output: Docker will include the custom build-time variables during the build process, allowing the Dockerfile to access and utilize them when executing build steps and commands.

Conclusion:

The docker build command is a versatile tool that allows users to build Docker images based on Dockerfiles. With the various options and arguments available, it offers great flexibility and customization during the image building process. Understanding and utilizing these use cases will enable efficient and streamlined image creation for containerized applications or services.