Create lab6-docker-build.md

Author: kchandan

lab6-docker-build.md

@@ -0,0 +1,115 @@
+### **Assignment: Understanding `CMD`, `RUN`, and `ENTRYPOINT`**
+
+**Objective**: Build a Docker image that demonstrates the use of `CMD`, `RUN`, and `ENTRYPOINT` directives effectively and understand their differences.
+
+---
+
+#### **Scenario**
+You are tasked with creating a Docker container for a simple Python application that performs mathematical calculations. The application should allow users to provide arguments at runtime to customize the calculations.
+
+---
+
+#### **Steps**
+
+1. **Create a Python Script**
+   Write a Python script (`calculator.py`) that can perform basic arithmetic operations (addition, subtraction, multiplication, and division).  
+   The script should accept arguments from the command line.
+
+   ```python
+   # calculator.py
+   import sys
+
+   def main():
+       if len(sys.argv) != 4:
+           print("Usage: python calculator.py <operation> <num1> <num2>")
+           sys.exit(1)
+
+       operation = sys.argv[1]
+       num1 = float(sys.argv[2])
+       num2 = float(sys.argv[3])
+
+       if operation == "add":
+           print(f"The result is: {num1 + num2}")
+       elif operation == "subtract":
+           print(f"The result is: {num1 - num2}")
+       elif operation == "multiply":
+           print(f"The result is: {num1 * num2}")
+       elif operation == "divide":
+           if num2 != 0:
+               print(f"The result is: {num1 / num2}")
+           else:
+               print("Error: Division by zero.")
+       else:
+           print("Error: Unsupported operation.")
+
+   if __name__ == "__main__":
+       main()
+   ```
+
+2. **Write the Dockerfile**
+   Create a Dockerfile that demonstrates the use of `RUN`, `CMD`, and `ENTRYPOINT`.
+
+   ```dockerfile
+   # Use Python as the base image
+   FROM python:3.10-slim
+
+   # Set the working directory inside the container
+   WORKDIR /app
+
+   # Copy the Python script into the container
+   COPY calculator.py /app/
+
+   # Install any dependencies (if needed)
+   # Using RUN to demonstrate execution at build time
+   RUN echo "Building the Docker image and setting up the environment..."
+
+   # Use ENTRYPOINT to ensure the script is always run with Python
+   ENTRYPOINT ["python", "calculator.py"]
+
+   # Set a default CMD for the container
+   CMD ["add", "10", "20"]
+   ```
+
+3. **Build the Docker Image**
+   Build the Docker image with a suitable tag.
+
+   ```bash
+   docker build -t calculator .
+   ```
+
+4. **Run the Container**
+   Run the container with and without overriding the default `CMD`.
+
+   - **Default CMD**:
+     ```bash
+     docker run calculator
+     ```
+     This should perform the default addition operation (10 + 20).
+
+   - **Override CMD**:
+     ```bash
+     docker run calculator subtract 30 10
+     ```
+     This should perform the subtraction operation (30 - 10).
+
+5. **Experiment with ENTRYPOINT**
+   Try overriding the `ENTRYPOINT` and see what happens:
+
+   ```bash
+   docker run --entrypoint echo calculator "Overriding entrypoint"
+   ```
+   This should override the `ENTRYPOINT` and print "Overriding entrypoint".
+
+6. **Document the Observations**
+   Answer the following questions:
+   - What happens when `CMD` is overridden?
+   - What happens when `ENTRYPOINT` is overridden?
+   - Why use `CMD` and `ENTRYPOINT` together?
+
+---
+
+#### **Deliverables**
+1. Dockerfile with properly configured `CMD`, `RUN`, and `ENTRYPOINT`.
+2. Screenshots or logs of running the container with various configurations.
+3. Written observations and answers to the provided questions.
+