How to reference embedded Docker resource files using file path URL?

Introduction

Embedded Docker resource files are files included in a Docker image rather than stored on the host file system or an external network location. These files can be useful for including configuration files, scripts, or other resources that are needed by the applications or processes running in the Docker container.

You can reference embedded Docker resource files in several different ways, including using file path URLs. This article explains what file path URLs are and how to use them to reference embedded Docker resource files. We will also provide tips and examples to help you use file path URLs effectively in your Dockerfiles and Docker Compose files.

Using file path URLs to reference embedded Docker resource files

File path URLs use the file:// protocol to reference a file on the local file system. In the context of Docker, file path URLs can reference embedded Docker resource files by specifying the path to the file within the Docker image.

Here is an example of using a file path URL to reference an embedded Docker resource file in a Dockerfile −

FROM ubuntu
COPY resources /app/resources
CMD ["cat", "file:///app/resources/config.txt"]

In this example, the resources directory is copied into the /app/resources directory in the Docker image. The CMD directive then runs the cat command to display the contents of the config.txt file, which is located at file:///app/resources/config.txt within the Docker image.

Example 

Here is an example of using a file path URL to reference an embedded Docker resource file in a Docker Compose file −

version: "3"
services:
   app:
      build: .
      command: ["cat", "file:///app/resources/config.txt"]
      volumes:
      - type: bind
        source: ./resources
        target: /app/resources

In this example, the resources directory on host is mounted as a volume in the /app/resources directory in the Docker container. The command field in the service definition then runs the cat command to display the contents of the config.txt file, which is located at file:///app/resources/config.txt within the Docker container.

Tips for using file path URLs to reference embedded Docker resource files

Here are some tips to help you use file path URLs effectively when referencing embedded Docker resource files −

• Make sure to use the correct path to the embedded resource file. The path should start with file:// and should be the path to the file within the Docker image or container.

• Be aware that file path URLs can only be used to reference files embedded in the Docker image or mounted as volumes in the Docker container. They cannot be used to reference files on the host file system or external network locations.

• If you are having trouble using file path URLs, try using absolute paths rather than relative paths. Absolute paths are more reliable and less prone to error, especially when using file path URLs.

Alternative methods for referencing embedded Docker resource files

Here are some alternative methods for referencing embedded Docker resource files −

• Using environment variables: You can use environment variables to specify the path to an embedded Docker resource file. For example, you can set the RESOURCE_FILE environment variable to the path of the resource file in the Dockerfile and then reference the environment variable in the CMD directive:

FROM ubuntu
ENV RESOURCE_FILE /app/resources/config.txt
COPY resources /app/resources
CMD ["cat", "$RESOURCE_FILE"]

• Using ADD or COPY directives: You can use the ADD or COPY directives in a Dockerfile to copy the resource file from the Docker image to a location on the host file system. This command allows you to reference the resource file using a regular file path on the host file system.

FROM ubuntu
ADD resources/config.txt /app/config.txt
CMD ["cat", "/app/config.txt"]

• Using volume mounts: If you are using Docker Compose, you can mount a host directory as a volume in the Docker container and then reference the resource file using a file path within the volume.

version: "3"
services:
   app:
      build: .
      command: ["cat", "/app/resources/config.txt"]
      volumes:
      - type: bind
        source: ./resources
        target: /app/resources

Conclusion

This article explained what file path URLs are and how to use them to reference embedded Docker resource files. We provided examples and tips to help you use file path URLs effectively in your Dockerfiles and Docker Compose files. We also discussed alternative methods for referencing embedded Docker resource files, including environment variables, ADD or COPY directives, and volume mounts.

Using file path URLs is a convenient way to reference embedded Docker resource files, especially if you want to avoid copying the resource files to the host file system or using environment variables. However, choosing the right method for referencing embedded Docker resource files is important based on your specific needs and requirements.