Building a Golang Docker Image with go.mod in a Sub Folder

Discover how to seamlessly build a `Golang` Docker image with `go.mod` located in a subfolder. Follow our clear breakdown for a stress-free setup process.
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This video is based on the question https://stackoverflow.com/q/65572263/ asked by the user 'iooi' ( https://stackoverflow.com/u/14718202/ ) and on the answer https://stackoverflow.com/a/65572406/ provided by the user 'colm.anseo' ( https://stackoverflow.com/u/1218512/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Build a Golang Docker Image with go.mod in a Sub Folder

When working on Go projects that involve Docker, a common headache arises: how to structure your application directories so that your Docker build processes smoothly. This is particularly tricky when your go.mod file resides in a subfolder, as is the case with many Go projects. If you’ve encountered the error about missing go.mod while trying to build your Docker image, you’re not alone. Let's unravel this problem and provide a clear solution to get your application running smoothly in Docker.

Understanding the Problem

Project Structure Overview

Your project is set up like this:

[[See Video to Reveal this Text or Code Snippet]]

Common Docker Error

When you attempt to build the Docker image from the my_app directory using the command docker build ., you may receive an error message stating that it cannot find the go.mod file located in the path /common/library/golang/go.mod. Here's the important part of the error:

[[See Video to Reveal this Text or Code Snippet]]

Why does this happen? The answer lies in the Docker context and how it handles file paths.

The Core Issue: Docker Context

When you run docker build ., Docker only has access to the current directory (in this case, my_app) and its child directories. It cannot access files in parent directories, which means it cannot see ../common/library/golang. This is the root cause of why the Docker build fails when it tries to download the modules specified in your go.mod file.

Solution: Restructure Your Docker Context

The solution to your problem is to restructure your directory setup so that the Docker context can access all necessary files. Follow these steps:

Step 1: Move the Dockerfile

Relocate the Dockerfile: Move the Dockerfile from the my_app directory up into the parent directory, my_project. Your new project structure will look like this:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Update Dockerfile References

Modify Your Dockerfile: Since the Dockerfile is now in a higher context, you will need to update any COPY commands to correctly reference the structure. Below is your updated Dockerfile:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Build the Image

Command to Build Your Docker Image: Now navigate to the my_project directory and run the build command:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By adjusting the location of your Dockerfile and making appropriate updates to its commands, your Go application should now successfully build within a Docker container. This approach ensures that Docker has access to all necessary files, including your Go modules located in subdirectories.

This restructuring technique not only solves the immediate issue but also enhances the organization of your project, making it easier for you and your team to maintain.

Feel free to leave any questions or comments, and happy coding!

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