Contents

Building a docker image

First, we need to create a file named Dockerfile with the following contents

# the base image
FROM openjdk:13

# sets up the working (this is where all subsequent commands will execute)
WORKDIR /app

# copy the files from the host inside the image
COPY target/greetings-webservice-0.0.1-SNAPSHOT.jar /app

# command executed when the container starts
CMD java -jar greetings-webservice-0.0.1-SNAPSHOT.jar

Let’s build our docker image

$ docker build -t greetings-ws .

The -t argument specifies the name and tag of the image and the . argument tells docker that it should run the command in the current directory.

Let’s look at the command output. The first thing you can notice are the 4 steps being executed:

Step 1/4 : FROM openjdk:13
13: Pulling from library/openjdk
Digest: sha256:22bda09e13ca9b3f951ad028b2c20b375d1792a55036097e7b9aa9eaede1a675
Status: Downloaded newer image for openjdk:13
 ---> e9ee535ba1b1
Step 2/4 : WORKDIR /app
 ---> Running in 236da3c84646
Removing intermediate container 236da3c84646
 ---> 83605da6c53f
Step 3/4 : COPY target/greetings-webservice-0.0.1-SNAPSHOT.jar /app
 ---> 1841358158c7
Step 4/4 : CMD java -jar greetings-webservice-0.0.1-SNAPSHOT.jar
 ---> Running in e65656cd8a33
Removing intermediate container e65656cd8a33
 ---> 99db7fcab4c3
Successfully built 99db7fcab4c3
Successfully tagged greetings-ws:latest

Let’s have a closer look at the command output

Because the docker client cannot find the openjdk image locally it downloads it from the registry and after it is downloaded the image digest is printed (you can verify this digest on Docker Hub, if you really want to double check docker’s integrity check). The last thing it prints at this stage is a Status.

Step 1/4 : FROM openjdk:13
13: Pulling from library/openjdk
Digest: sha256:22bda09e13ca9b3f951ad028b2c20b375d1792a55036097e7b9aa9eaede1a675
Status: Downloaded newer image for openjdk:13

Docker image layers

The building blocks of a docker image are the image layers. A layer contains the differences between the previous and the current layer. All the layers are read-only, except the last one.

After each build step you can notice that docker prints Removing intermediate container <hash>. In order to execute some commands docker needs to create intermediate containers. When the build step execution is finished the intermediate container is removed and replaced by a read-only layer (intermediate).

The layers that will make up the image are printed on the lines starting with --->.

Step 2/4 : WORKDIR /app
 ---> Running in 236da3c84646
Removing intermediate container 236da3c84646
 ---> 83605da6c53f
Step 3/4 : COPY target/greetings-webservice-0.0.1-SNAPSHOT.jar /app
 ---> 1841358158c7
Step 4/4 : CMD java -jar greetings-webservice-0.0.1-SNAPSHOT.jar
 ---> Running in e65656cd8a33
Removing intermediate container e65656cd8a33
 ---> 99db7fcab4c3

The output

The last 2 lines print the last layer (container layer) hash and the image name and tag.

Successfully built 99db7fcab4c3
Successfully tagged greetings-ws:latest

Working with docker images

If one wants to see how a docker image was built, a very useful command is:

$ docker history 99db7fcab4c3

The output will show information about every layer

IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
99db7fcab4c3        3 hours ago         /bin/sh -c #(nop)  CMD ["/bin/sh" "-c" "java…   0B                  
1841358158c7        3 hours ago         /bin/sh -c #(nop) COPY file:b852d9538176de45…   38.7MB              
83605da6c53f        3 hours ago         /bin/sh -c #(nop) WORKDIR /app                  0B                  
e9ee535ba1b1        6 days ago          /bin/sh -c #(nop)  CMD ["jshell"]               0B                  
<missing>           6 days ago          /bin/sh -c set -eux;   curl -fL -o /openjdk.…   330MB               
<missing>           6 days ago          /bin/sh -c #(nop)  ENV JAVA_SHA256=2e0171654…   0B                  
<missing>           6 days ago          /bin/sh -c #(nop)  ENV JAVA_URL=https://down…   0B                  
<missing>           6 days ago          /bin/sh -c #(nop)  ENV JAVA_VERSION=13.0.1      0B                  
<missing>           6 days ago          /bin/sh -c #(nop)  ENV PATH=/usr/java/openjd…   0B                  
<missing>           6 days ago          /bin/sh -c #(nop)  ENV JAVA_HOME=/usr/java/o…   0B                  
<missing>           6 days ago          /bin/sh -c #(nop)  ENV LANG=en_US.UTF-8         0B                  
<missing>           6 days ago          /bin/sh -c set -eux;  yum install -y   gzip …   42.2MB              
<missing>           6 days ago          /bin/sh -c #(nop)  CMD ["/bin/bash"]            0B                  
<missing>           6 days ago          /bin/sh -c #(nop) ADD file:c8bbabb7270612c9e…   118MB               
<missing>           15 months ago       /bin/sh -c #(nop)  MAINTAINER Oracle Linux P…   0B

Another useful command is docker images. This command will display all the images present on the local computer.

REPOSITORY       TAG        IMAGE ID            CREATED             SIZE
greetings-ws     latest     99db7fcab4c3        2 seconds ago       529MB
openjdk          13         e9ee535ba1b1        6 days ago          491MB