Environment files (.env) in Docker - Mini Project: Build & Apply

Using Environment Files (.env) with Docker

📖 Scenario: You are creating a simple Docker container that needs to use environment variables for configuration. Instead of hardcoding these variables inside the Dockerfile or docker run command, you will use an environment file (.env) to keep your settings organized and easy to change.This is like writing down your secret recipe on a note instead of memorizing it every time you cook. It helps keep things clean and safe.

🎯 Goal: Build a Docker setup that reads environment variables from a .env file and uses them inside a container.You will create the .env file, configure Docker to use it, and then run a container that prints the environment variables.

📋 What You'll Learn

Create a .env file with specific environment variables

Write a simple Dockerfile that uses an environment variable

Configure Docker Compose to load the .env file

Run the container and display the environment variables

💡 Why This Matters

🌍 Real World

Environment files are used in real projects to keep configuration separate from code. This makes it easy to change settings without rebuilding images or changing code.

💼 Career

Knowing how to use <code>.env</code> files with Docker is a key skill for DevOps engineers and developers working with containerized applications.

1

Create the .env file with environment variables

Create a file named .env with these exact lines:
APP_NAME=MyApp
APP_PORT=8080

Docker

# Create a .env file with APP_NAME=MyApp and APP_PORT=8080
# Your code here

Hint

Use a text editor to create a file named .env and add the two lines exactly as shown.

2

Write a simple Dockerfile that uses the environment variables

Create a Dockerfile with these exact lines:
FROM alpine
ENV APP_NAME=$APP_NAME
ENV APP_PORT=$APP_PORT
CMD ["sh", "-c", "echo Running $APP_NAME on port $APP_PORT"]

Docker

APP_NAME=MyApp
APP_PORT=8080

# Create a Dockerfile with FROM alpine and ENV variables
# Your code here

Hint

Write a Dockerfile starting with FROM alpine. Use ENV to set APP_NAME and APP_PORT from the environment variables. Use CMD to print a message using these variables.

3

Create a docker-compose.yml file to load the .env file

Create a docker-compose.yml file with these exact contents:

version: '3.8'
services:
  app:
    build: .
    env_file:
      - .env

Docker

APP_NAME=MyApp
APP_PORT=8080

FROM alpine
ENV APP_NAME=$APP_NAME
ENV APP_PORT=$APP_PORT
CMD ["sh", "-c", "echo Running $APP_NAME on port $APP_PORT"]

# Create docker-compose.yml with version 3.8, service app, build ., and env_file .env
# Your code here

Hint

Create a docker-compose.yml file that builds the current directory and loads the .env file using env_file.

4

Run the container and display the environment variables

Run docker-compose up --build in the terminal and observe the output. The container should print:
Running MyApp on port 8080

Docker

APP_NAME=MyApp
APP_PORT=8080

FROM alpine
ENV APP_NAME=$APP_NAME
ENV APP_PORT=$APP_PORT
CMD ["sh", "-c", "echo Running $APP_NAME on port $APP_PORT"]

version: '3.8'
services:
  app:
    build: .
    env_file:
      - .env

# Run docker-compose up --build and check output
# Your code here

Hint

Use the command docker-compose up --build to build and run the container. The output should show the message with the environment variables.

Practice

1. What is the main purpose of a .env file in Docker?

easy

A. To list all running Docker containers

B. To write Docker commands for building images

C. To store environment variables outside the Dockerfile for easy configuration

D. To define network settings for Docker

Solution

1. Step 1: Understand the role of .env files

   .env files hold environment variables separately from code or Dockerfiles, making it easy to change settings without editing code.

2. Step 2: Identify the correct purpose in Docker context

   Docker uses .env files to load environment variables into containers, not for commands, container lists, or network settings.

3. Final Answer:

   To store environment variables outside the Dockerfile for easy configuration -> Option C

4. Quick Check:

   .env files = environment variables storage [OK]

Hint: Think: .env means environment variables outside code [OK]

Common Mistakes:

• Confusing .env with Dockerfile commands
• Thinking .env lists containers or networks
• Assuming .env files run commands

2. Which of the following is the correct syntax for a line in a .env file?

easy

A. MY_VAR = value

B. MY_VAR: value

C. MY_VAR=>value

D. MY_VAR=value

Solution

1. Step 1: Recall .env file syntax rules

   Each line must be a key-value pair with no spaces around the equal sign, like KEY=value.

2. Step 2: Check each option

   MY_VAR=value uses no spaces and an equal sign, which is correct. The other options use spaces, colon, or arrow separators, which are incorrect.

3. Final Answer:

   MY_VAR=value -> Option D

4. Quick Check:

   KEY=value with no spaces [OK]

Hint: No spaces around = in .env lines [OK]

Common Mistakes:

• Adding spaces around equal sign
• Using colon or arrow instead of equal sign
• Using quotes unnecessarily

3. Given a .env file with content:

APP_NAME=MyApp
APP_PORT=8080

and a Docker Compose service using env_file: .env, what will be the value of APP_PORT inside the container?

medium

A. 8080

B. Not set

C. APP_PORT

D. MyApp

Solution

1. Step 1: Understand env_file usage in Docker Compose

   Docker Compose loads variables from the .env file and sets them as environment variables inside the container.

2. Step 2: Identify the value of APP_PORT

   The .env file sets APP_PORT=8080, so inside the container, APP_PORT will be "8080" as a string.

3. Final Answer:

   8080 -> Option A

4. Quick Check:

   APP_PORT = 8080 from .env [OK]

Hint: env_file loads variables exactly as key=value [OK]

Common Mistakes:

• Confusing variable names with values
• Assuming variables are not set inside container
• Mixing up APP_NAME and APP_PORT values

4. You have a .env file with the line DB_PASS=secret pass. When you run your Docker container, the environment variable DB_PASS is not set correctly. What is the likely problem?

medium

A. The space in the value causes parsing issues; values should not contain spaces without quotes

B. The key DB_PASS is invalid because it contains uppercase letters

C. The .env file must be named env.txt to work

D. Docker does not support environment variables from .env files

Solution

1. Step 1: Check .env file value format rules

   Values with spaces must be quoted or use no spaces; otherwise, parsing breaks.

2. Step 2: Identify the issue with the given line

   "secret pass" has a space but no quotes, so Docker reads only "secret" or fails to parse correctly.

3. Final Answer:

   The space in the value causes parsing issues; values should not contain spaces without quotes -> Option A

4. Quick Check:

   Spaces in values need quotes or no spaces [OK]

Hint: No spaces in values unless quoted in .env [OK]

Common Mistakes:

• Ignoring spaces in values
• Thinking uppercase keys are invalid
• Renaming .env file incorrectly

5. You want to use a .env file to set environment variables for multiple Docker Compose services, but one service needs a different value for API_URL. How can you handle this situation?

hard

A. Put all variables in one .env file and override the variable in the Dockerfile

B. Use multiple .env files and specify the correct one per service with env_file in each service definition

C. Use only one .env file and change the variable manually before running Docker Compose

D. Docker Compose does not support different env files per service

Solution

1. Step 1: Understand env_file usage per service

   Docker Compose allows specifying env_file separately for each service to load different environment variables.

2. Step 2: Apply multiple .env files for different services

   Create separate .env files with different API_URL values and assign them to each service using env_file.

3. Final Answer:

   Use multiple .env files and specify the correct one per service with env_file in each service definition -> Option B

4. Quick Check:

   Multiple env_files per service = different variables [OK]

Hint: Use separate env_file per service for different variables [OK]

Common Mistakes:

• Trying to override variables only in Dockerfile
• Editing one .env file manually each time
• Believing Docker Compose can't use multiple env files