8.4 Using Dockerfile

How to create a container

Hello world

Create a file called Dockerfile where we will print

FROM alpine
CMD ["echo", "Hell on World!"]

# clone repository
# change directory to docker_examples/hello_world
# get location by running explorer.exe .
# copy the location from explorer
# change directory to the location

# build the created image with a tag (-t) hello from current location (.)
docker build -t hello .

# instantiate a container based on created image using tag hello
docker run hello

# instantiate a container based on created image using tag hello and giving the container a name
docker run --name HelloWorld hello

# instantiate a container based on created image using tag hello and remove it after run(-rm)
docker run --rm hello

Deploy web server

Create a file index.html

<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Introduction to DevOps - Docker</title>
<style>
    html, body {
    text-align: center;
    }
</style>
</head>

<body>
<h1>Introduction to DevOps - Docker</h1>
<h2>Web server deployed successfully!</h2>
</body>
</html>

And a file Dockerfile

FROM nginx:stable-alpine

# Change our working directory to the root of nginx web host
WORKDIR /usr/share/nginx/html

# Copy in the the index.html
COPY index.html /usr/share/nginx/html/

# clone repository using git clone repo_url
# change directory to docker_examples/hello_world
# get location by running explorer.exe .
# copy the location from explorer
# change directory to the location

docker build -t web_server .

# build the created image with a tag (-t) hello from current location (.)
docker container run -p 8080:80 --name web_server -d web_server

Building a Dockerfile

Before we construct our Dockerfile, you need to understand what makes up the file. This will be a text file, named Dockerfile, that includes specific keywords that dictate how to build a specific image. The specific keywords you can use in a file are:

  • ADD copies the files from a source on the host into the container’s own filesystem at the set destination.

  • ENV sets environment variables.

  • EXPOSE associates a specific port to enable networking between the container and the outside world.

  • FROM defines the base image used to start the build process.

  • USER sets the UID (or username) which is to run the container.

  • VOLUME is used to enable access from the container to a directory on the host machine.

  • WORKDIR sets the path where the command, defined with CMD, is to be executed.

  • LABEL allows you to add a label to your docker image.

  • RUN - RUN instruction allows you to install your application and packages required for it. It executes any commands on top of the current image and creates a new layer by committing the results. Often you will find multiple RUN instructions in a Dockerfile.

  • CMD - can be used for executing a specific command within the container.CMD instruction allows you to set a default command, which will be executed only when you run container without specifying a command. If Docker container runs with a command, the default command will be ignored. If Dockerfile has more than one CMD instruction, all but last CMD instructions are ignored. The CMD can be overridden when starting a container with docker run image_name other_command example docker run fedora bash.

  • ENTRYPOINT is also closely related to CMD and can modify the way a container is started from an image, sets a default application to be used every time a container is created with the image.

FROM ubuntu:latest
LABEL YOUR_NAME YOUR_EMAIL

RUN apt-get -y update
RUN apt-get -y upgrade
RUN apt-get install -y tree

CMD tree -d -L 2 /

to build the image we run docker build -t getting_tree . and to run docker docker run getting_tree