Now, let’s try to go with a little complex example. Lets say, we want to build a container from a spring boot application

Here is the docker file, We will first try to understand the docker file, and then look at what happens when we run it.

cd ~/workspace

git clone [git@gitlab.crio.do](mailto:git@gitlab.crio.do):public_content/spring-starter.git

cd spring-starter

This spring starter has a file named Dockerfile which contains a code like below. Try to browse through it, and see what it does.

FROM gradle:jdk11


ENV DEBIAN_FRONTEND=noninteractive

RUN apt-get update && apt-get -y upgrade

RUN apt-get -y install git redis-server wget


RUN apt-get install -y gnupg

RUN wget -qO - https://www.mongodb.org/static/pgp/server-4.2.asc | apt-key add -

RUN wget -qO - https://www.mongodb.org/static/pgp/server-4.2.asc | apt-key add -

RUN echo "deb https://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/4.2 multiverse" | tee /etc/apt/sources.list.d/mongodb-org-4.2.list

RUN apt-get update

RUN apt-get install -y mongodb-org


USER root


RUN mkdir code

COPY . /code


RUN cd /code && ./gradlew bootjar


CMD /code/start.sh

Let's take a look at the steps involved in building this image.

1. Baseline for this image is gradle:jdk11 image available on docker hub. The details are available here

2. Next is ARG command, that sets up a temporary environment variable which enabled non-interactive install

3. From step 3-9 are few commands that install required softwares on the machine.

4. Step 10 is changing the underlying user to ROOT

5. Step 11 is a COPY command that can copy files from Host filesystem to guest filesystem

6. Step 12 and 13 are about making another directory /code and copying the contents of current directory on host os to /code on the guest os.

7. Step 14 is again another command that actually builds the copied spring application and generates a bootable jar that can be used as a server.

8. The last line is about defining entrypoint. While you can say that everything from Step 1-14 attributes to the container image, Step 15 is executed when you start the container for the first time. When you start the container, the command mentioned as CMD will be executed.

Let's try to create the image now.

docker build -t spring-starter .

Now go to hub.docker.com, We have the same image pushed over there. https://hub.docker.com/repository/docker/criodo/spring-starter/

Click on tags.

Click on latest and it will show the details of the docker image.

Click on the commands shown there to see the details of each command

Layers is nothing but something similar to git commit. When you execute any command during docker image creation, the changes are recorded as a layer, and not persisted on the image.

When a container starts, these changes are "Computed" based on layer information, and the filesystem is derived.

By now, you know that containers share the file resources with host operating system, including kernel and process space.