docker-compose.yml launched with docker compose --env-file .myenv. Why isn't the vars in .myenv accessible to docker-compose.yml entrypoint

When using Docker Compose with an --env-file option, it’s essential to understand how environment variables are handled within your application. If you find that the variables defined in your .myenv file are not accessible in the entrypoint of your docker-compose.yml, several factors could be influencing this behavior. Below is a detailed analysis of the situation.

Understanding the Environment Variable Scope in Docker Compose

Environment Variable Definition

1. Environment Variables in docker-compose.yml: The env_file directive in a docker-compose.yml file allows you to specify an environment file that Docker Compose will read and pass to the containers. It does work, but it does not perform variable substitution in the instructions within the docker-compose.yml itself.

2. Using --env-file: When you launch a Docker Compose project with docker compose --env-file .myenv, it loads the specified environment variables into the environment of the docker-compose command itself. These variables are not automatically passed down to the containers or accessible from the entrypoint.

Variable Accessibility Issues

1. Entrypoint Limitations: Variables loaded from an env_file are available to the services defined in your docker-compose.yml. However, if your entrypoint is trying to access these variables at the build stage or outside the execution context of the container, they will not be available.

2. Override Behavior: If you also have environment variables defined directly within your docker-compose.yml under the environment section, those will take precedence over variables defined in the .myenv file.

3. Container Execution Context: The entrypoint script runs in the context of the container, not the host where the docker-compose command was executed. If a variable is defined in .myenv but not exported properly when the container starts, it will not be accessible in the entrypoint script.

Best Practices to Ensure Access

1. Direct Inclusion in docker-compose.yml: To ensure that variables are accessible in the entrypoint, define them directly in docker-compose.yml:

   services:
     myservice:
       image: myimage
       env_file:
         - .myenv
   

2. Entrypoint Script Adaptation: If your entrypoint script needs to access these variables, consider using the printenv command or similar to debug:

   # In your entrypoint.sh
   echo "MY_VAR=${MY_VAR}"  # This helps you see if the variable is set.
   

3. Alternative Methods: If it's crucial for your deployment that these variables are available early, consider directly injecting them into the environment through the environment block:

   environment:
     MY_VAR: ${MY_VAR}
   

Example Configuration

Here’s an example of how your configuration might look:

version: '3.8'

services:
  app:
    image: myapp
    env_file:
      - .myenv
    entrypoint: ["./entrypoint.sh"]

Conclusion

If you find that the environment variables from .myenv are not accessible in your Docker Compose entrypoint, verify how and where those variables are defined and accessed. Ensure that your docker-compose.yml properly includes the .myenv file, and make adjustments to your entrypoint script accordingly to handle or debug the availability of your environment variables.

For deeper insights into managing environment variables in Docker, you can reference sources such as Configu and discussions in forums like Docker Community.