added day 31 tasks

Author: LondheShubham153

2026/day-31/README.md

@@ -0,0 +1,95 @@
+# Day 31 – Dockerfile: Build Your Own Images
+
+## Task
+Today's goal is to **write Dockerfiles and build custom images**.
+
+This is the skill that separates someone who uses Docker from someone who actually ships with Docker.
+
+---
+
+## Expected Output
+- A markdown file: `day-31-dockerfile.md`
+- All Dockerfiles you create
+
+---
+
+## Challenge Tasks
+
+### Task 1: Your First Dockerfile
+1. Create a folder called `my-first-image`
+2. Inside it, create a `Dockerfile` that:
+   - Uses `ubuntu` as the base image
+   - Installs `curl`
+   - Sets a default command to print `"Hello from my custom image!"`
+3. Build the image and tag it `my-ubuntu:v1`
+4. Run a container from your image
+
+**Verify:** The message prints on `docker run`
+
+---
+
+### Task 2: Dockerfile Instructions
+Create a new Dockerfile that uses **all** of these instructions:
+- `FROM` — base image
+- `RUN` — execute commands during build
+- `COPY` — copy files from host to image
+- `WORKDIR` — set working directory
+- `EXPOSE` — document the port
+- `CMD` — default command
+
+Build and run it. Understand what each line does.
+
+---
+
+### Task 3: CMD vs ENTRYPOINT
+1. Create an image with `CMD ["echo", "hello"]` — run it, then run it with a custom command. What happens?
+2. Create an image with `ENTRYPOINT ["echo"]` — run it, then run it with additional arguments. What happens?
+3. Write in your notes: When would you use CMD vs ENTRYPOINT?
+
+---
+
+### Task 4: Build a Simple Web App Image
+1. Create a small static HTML file (`index.html`) with any content
+2. Write a Dockerfile that:
+   - Uses `nginx:alpine` as base
+   - Copies your `index.html` to the Nginx web directory
+3. Build and tag it `my-website:v1`
+4. Run it with port mapping and access it in your browser
+
+---
+
+### Task 5: .dockerignore
+1. Create a `.dockerignore` file in one of your project folders
+2. Add entries for: `node_modules`, `.git`, `*.md`, `.env`
+3. Build the image — verify that ignored files are not included
+
+---
+
+### Task 6: Build Optimization
+1. Build an image, then change one line and rebuild — notice how Docker uses **cache**
+2. Reorder your Dockerfile so that frequently changing lines come **last**
+3. Write in your notes: Why does layer order matter for build speed?
+
+---
+
+## Hints
+- Build: `docker build -t name:tag .`
+- The `.` at the end is the build context
+- `COPY . .` copies everything from host to container
+- Nginx serves files from `/usr/share/nginx/html/`
+
+---
+
+## Submission
+1. Add your Dockerfiles and `day-31-dockerfile.md` to `2026/day-31/`
+2. Commit and push to your fork
+
+---
+
+## Learn in Public
+Share your custom Docker image or Nginx screenshot on LinkedIn.
+
+`#90DaysOfDevOps` `#DevOpsKaJosh` `#TrainWithShubham`
+
+Happy Learning!
+**TrainWithShubham**