Debakar Roy
DevOps Software Development Automation Infrastructure Containerization Docker

Day 28: Docker - Building and managing images

1 min read

Building a Docker image

A Docker image is a lightweight, standalone, executable software package with everything needed to run a piece of software: code, runtime, system tools, libraries, and dependencies. Images are built from the instructions in a Dockerfile. Here is one for a FastAPI app:

# Base image: the official Python 3.9-slim image is our starting point.
FROM python:3.9-slim

# Set the working directory inside the container; later commands run here.
WORKDIR /app

# Copy in requirements first so dependency installation can be cached.
COPY requirements.txt .

# Install the Python dependencies.
RUN pip install --no-cache-dir -r requirements.txt

# Copy the rest of the application code into the container.
COPY . .

# Expose port 7400 for the FastAPI application.
EXPOSE 7400

# Default command: serve the app in main.py with uvicorn on 0.0.0.0:7400.
CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "7400"]

With this Dockerfile in your project directory, build the image:

docker build -t my-fastapi-app .

Then run a container from it:

docker run -p 7400:7400 my-fastapi-app

This binds port 7400 of the container to port 7400 of the host, so the app is reachable at http://localhost:7400.

Managing Docker images

Essential commands for image management:

  1. docker tag — adds a new tag (a human-readable label) to an existing image.
  2. docker images — lists local images with their ID, repository, tag, size, and creation date.
  3. docker push — uploads a local image to a remote registry so others can use it.
  4. docker pull — downloads an image from a registry like Docker Hub to your machine.
  5. docker rmi — removes a local image by ID or repository name, cleaning up unused ones.

Example

# Tag the FastAPI image as "latest"
$ docker tag my-fastapi-app my-fastapi-app:latest

# List local images with repository, tag, ID, creation date, and size
$ docker images
REPOSITORY            TAG         IMAGE ID       CREATED       SIZE
myjenkins-blueocean   2.387.3-1   3eb5689ff763   6 days ago    790MB
my-fastapi-app        latest      67cdycd99743   2 hours ago   150MB
docker                dind        85cdec499643   8 days ago    323MB
postgres              latest      f14b0d96cff9   2 weeks ago   379MB
vault                 latest      1fa9c3f0708d   4 weeks ago   186MB

# Fetch the latest nginx image from Docker Hub
$ docker pull nginx:latest
latest: Pulling from library/nginx
f03b40093957: Pull complete
eed12bbd6494: Pull complete
fa7eb8c8eee8: Pull complete
7ff3b2b12318: Pull complete
0f67c7de5f2c: Pull complete
831f51541d38: Pull complete
Digest: sha256:af296b188c7b7df99ba960ca614439c99cb7cf252ed7bbc23e90cfda59092305
Status: Downloaded newer image for nginx:latest
docker.io/library/nginx:latest

# Push a local image to a remote registry
$ docker push debakarr/myjenkins-blueocean:latest
The push refers to repository [docker.io/debakarr/myjenkins-blueocean]
ebe100ea75f2: Pushed
aa20ea415e9b: Pushed
dd7368f0b1ad: Pushed
6a2689aa4ed2: Pushed
9dfe2887b847: Pushed
ddabbb0c670a: Mounted from jenkins/jenkins
e2cf45412e4b: Mounted from jenkins/jenkins
12e639a5a7ea: Mounted from jenkins/jenkins
ba66619e71de: Mounted from jenkins/jenkins
d90c97aff719: Mounted from jenkins/jenkins
67ac8a362561: Mounted from jenkins/jenkins
6a45b66c291b: Mounted from jenkins/jenkins
56b018284c58: Mounted from jenkins/jenkins
7b8f9d0c670c: Mounted from jenkins/jenkins
3811da883895: Mounted from jenkins/jenkins
7de19ea2fded: Mounted from jenkins/jenkins
3fb287e008f8: Mounted from jenkins/jenkins
ae56c0c5405b: Mounted from jenkins/jenkins
latest: digest: sha256:97ce065841e7e33ac746396a46c4ef5865a772c2bd5b5651aa96b83951148f00 size: 4092

# Remove a local image
$ docker rmi nginx
Untagged: nginx:latest
Untagged: nginx@sha256:af296b188c7b7df99ba960ca614439c99cb7cf252ed7bbc23e90cfda59092305
Deleted: sha256:f9c14fe76d502861ba0939bc3189e642c02e257f06f4c0214b1f8ca329326cda
Deleted: sha256:419f8948c50c723f2a5ac74428af3d804b5d0079d6df8f7f827663cf10cbc366
Deleted: sha256:1030aac4f1a8096ed58d3d4a2df55dd1b1b27d919ad156d97ad1f68081d0051a
Deleted: sha256:7d90b49d96c3036539ef144ecc27c01de03902d8ea166a0f7f827663cf10cbc366
Deleted: sha256:551acb210764654af31b6cd51adaa74edc9a202587c3395fe0e9f95a2e097f8b
Deleted: sha256:3c530958db4c75c6fb409f339367aaf9a1e163c84718c035d4b09bebc83f43e7
Deleted: sha256:8cbe4b54fa88d8fc0198ea0cc3a5432aea41573e6a0ee26eca8c79f9fbfa40e3

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