We can think of Dockerfile as a recipe for creating a new image. It contains step-by-step instructions and some other configurations for building the image. But in general, it usually contains the following key steps:
- Define a base image: This step usually uses a
FROMinstruction to set a base image for subsequent instructions. - Run commands to configure the image: This step uses various instructions such as
RUN,ADD,COPYto install dependencies, copy files, and configure env variables within the image. - Specify a startup command: This step usually uses the
CMDinstruction to define the startup command that will be executed when a container is started from the image.
Example
The following Dockerfile uses alpine as the base image, installs redis for the image, and specifies to run redis-server whenever someone starts a container from this image.
# Using an existing image as a base
FROM alpine
# Download and install redis as a dependency
RUN apk add --update redis
# Tell the image what to do when it starts as a container
CMD [ "redis-server" ]Once the Dockerfile is completed, we build the image with the docker build command and run the generated image using docker run.
docker build .
# [+] Building 0.1s (6/6) FINISHED
# => [internal] load build definition from Dockerfile
# => => transferring dockerfile: 100B
# => [internal] load metadata for docker.io/library/alpine:latest
# => [internal] load .dockerignore
# => => transferring context: 2B
# => [1/2] FROM docker.io/library/alpine:latest
# => CACHED [2/2] RUN apk add --update redis
# => exporting to image
# => => exporting layers
# => => writing image sha256:dee0eaedf501b9b30875f44680225a44e766c727044c6c0412c1f039668508b3
docker run dee0eaed
# * Ready to accept connections tcpYou can visit Docker build process to learn the details of the Docker build and caching mechanism.
References