Nicholas Dille

Automation, DevOps and Containerization

© 2001-2024. All rights reserved.

Custom images

Custom images

Custom behaviour

Based on existing image

Adds tools and functionality

Simple but sufficient language

Create Dockerfile:

$ cat >Dockerfile <<EOF
FROM ubuntu:20.04
RUN apt update && apt -y install nginx
EOF

Build image from Dockerfile:

docker build --tag myimage .

–

Dockerfile 1/4

Specify first process to run

$ cat >Dockerfile <<EOF
FROM openjdk:11-jre
CMD java -version
EOF
$ docker build --tag cmd .
$ docker run cmd

Use variables

Many containerized services use environment variables for configuration:

$ cat >Dockerfile <<EOF
FROM postgres
ENV POSTGRES_PASSWORD=foobar
EOF
$ docker build --tag my_postgres .
$ docker run my_postgres

–

Dockerfile 2/4

Use build arguments

$ cat >Dockerfile <<EOF
FROM ubuntu
ARG VERSION=1.24.1
RUN apt-get update \
 && apt-get -y install --no-install-recommends curl ca-certificates
RUN curl --location --output /usr/local/bin/docker-compose https://github.com/docker/compose/releases/download/${VERSION}/docker-compose-`uname -s`-`uname -m` \
 && chmod +x /usr/local/bin/docker-compose
CMD /usr/local/bin/docker-compose
EOF
$ docker build --build-arg VERSION=1.29.2 --tag docker-compose .
$ docker run docker-compose

–

Dockerfile 3/4

Shell versus Exec Notation

Applies to RUN, CMD, ENTRPOINT

Shell notation wraps command in shell:

FROM openjdk:11-jre
CMD java -version

…is equivlent to…

docker run -it openjdk:11-jre sh -c 'java version'

Exec notation starts process without shell:

FROM openjdk:11-jre
CMD [ "java", "-version" ]

Equivalent to…

docker run -it openjdk:11-jre java -version

–

Dockerfile 4/4

ENTRYPOINT

ENTRYPOINT hardcodes the process to run

CMD defines parameters

Command line overrides CMD

$ cat >Dockerfile <<EOF
FROM openjdk:11-jre
ENTRYPOINT [ "java" ]
CMD [ "-version" ]
EOF
$ docker build --tag java .
$ docker run -it java -help

–

Tagging images

Create Dockerfile

$ cat >Dockerfile <<EOF
FROM ubuntu
ARG VERSION=1.29.2
RUN apt-get update \
 && apt-get -y install --no-install-recommends curl ca-certificates
RUN curl --location --output /usr/local/bin/docker-compose https://github.com/docker/compose/releases/download/${VERSION}/docker-compose-`uname -s`-`uname -m` \
 && chmod +x /usr/local/bin/docker-compose
CMD /usr/local/bin/docker-compose
EOF

Build image and assign multiple tags:

docker build . \
    --tag docker-compose:1.29.2 \
    --tag docker-compose:1.29 \
    --tag docker-compose:1

Assign alternative names:

docker tag docker-compose:1.29.2 docker-compose:latest