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Writing a Dockerfile

Guidelines for writing a Dockerfile.

These are the main guidelines for writing Dockerfiles:

  1. Building your Dockerfile should result a light docker image. It is recommended to have an image size of no more than 100Mb. This reduces build and shipping time and makes the build & deployment run faster. It is very important to invest time in this and try to reduce the image size as much as possible.

  2. Use an ARG in your FROM line. This will allow you to:

  3. Control the base image from the CI

  4. Update a project or all projects by updating a Variable.

Examples

DockerFile

ARG NGINX_VER
FROM nginx:${NGINX_VER}-alpine

In the .gitlab-ci.yml file set the Base image tag you would like to use:

variables:
  NGINX_VER: "1.17.8"

In the build stage pass it as a build argument with the build option --build-arg:

build:
  stage: build
  script:
    - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $CI_REGISTRY
    - docker build --build-arg NGINX_VER=${NGINX_VER} -t $DEV_TAG .
    - docker tag $DEV_TAG $LATEST_DEV
    - docker push $DEV_TAG
    - docker push $LATEST_DEV
  tags:
    - generic
  except:
    - tags

After installing packages in your Dockerfile, clean it up.

RUN apk add --update \
    python \
    python-dev \
    py-pip \
    build-base \
  && pip install virtualenv \
  && rm -rf /var/cache/apk/*

Depending on your FROM image it is also possible to run apk add without cache.

RUN apk --no-cache add nginx

This avoids the need to use

--update and remove /var/cache/apk/*
When done installing packages.

Using multi-stage builds

Below there are two examples of the same build

  • The 1st one is hard to maintain & read.

  • The second example is readable and allows you to build the image with 2 stages, copying the artifacts of the 1st stage to the 2nd build stage.

  • This is all done with a single Dockerfile.

DO NOT USE THIS !

This is not good practice.

FROM golang:1.7.3
WORKDIR /go/src/github.com/alexellis/href-counter/
COPY app.go .
RUN go get -d -v golang.org/x/net/html \
  && CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .

USE THIS

This is better way to create the final image. Docker

FROM golang:1.7.3
WORKDIR /go/src/github.com/alexellis/href-counter/
RUN go get -d -v golang.org/x/net/html  
COPY app.go .
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .

FROM alpine:latest  
RUN apk --no-cache add ca-certificates
WORKDIR /root/
COPY --from=0 /go/src/github.com/alexellis/href-counter/app .
CMD ["./app"]

There is great documentation at docker website.

Dockerfiles

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