Here’s a simple and clear explanation of each important Dockerfile instruction with easy-to-remember examples:
🏗️ 1. FROM
Purpose: Sets the base image for your Docker image.
Example
FROM mcr.microsoft.com/dotnet/aspnet:8.0
✔️ Think: "Start from this base image first."
📁 2. WORKDIR
Purpose: Sets the working directory (like cd in Linux).
Example
WORKDIR /app
✔️ Think: "Go to the /app folder before running commands."
📋 3. COPY
Purpose: Copies files/folders from your computer to the Docker image.
Example
COPY . .
✔️ Think: "Copy everything from my project folder into the container's /app (if WORKDIR is set)."
⚙️ 4. RUN
Purpose: Runs commands during build time (e.g., install packages).
Example
RUN dotnet restore
✔️ Think: "Run this command when building the image."
🌐 5. EXPOSE
Purpose: Tells Docker which port the app inside the container will use.
Example
EXPOSE 5000
✔️ Think: "This app listens on port 5000."
🚪 6. ENTRYPOINT
Purpose: Defines the main starting command when the container runs.
Example
ENTRYPOINT ["dotnet", "MyApp.dll"]
✔️ Think: "When this container starts, run this command."
🎯 Small Complete Example (to remember easily)
FROM mcr.microsoft.com/dotnet/aspnet:8.0 # Use .NET base image
WORKDIR /app # Go to /app folder
COPY . . # Copy all files into /app
RUN dotnet restore # Restore packages during build
EXPOSE 5000 # App will use port 5000
ENTRYPOINT ["dotnet", "MyApp.dll"] # Start the app when container runs
Quick Memory Trick 🧠
F - FROM (Base image)
W - WORKDIR (Set working folder)
C - COPY (Copy files into image)
R - RUN (Run build-time command)
E - EXPOSE (Open a port)
E - ENTRYPOINT (Start the app)
FWCREE – Just remember this order. 🔑