Build a Container with a Dockerfile

Creating the Dockerfile

If a container does not already exist for your application, one can be built for your device.

It is common to create images from a working directory which holds the Dockerfile and any supporting files. This may be a version controlled directory to facilitate sharing.

$ mkdir demo-docker && cd demo-docker

There are many examples of building containers using a Dockerfile. A simple Dockerfile will contain some of the following elements:

  • The FROM line indicates the base, or starting, container, such as a latest Fedora image. This image will be pulled if it is not already available locally. Specify details for which image the same as you would with a podman pull command.

  • Creates layers with each RUN command. Try to minimize the number of layers with multiple commands on the same line using && between commands. Also include any cleanup commands such as dnf clean all to reduce the final image size.

  • Copy content from the working directory into the container.

  • Specify any ports to listen on with EXPOSE

  • Start your application

    • CMD can be over written with podman run command

    • ENTRYPOINT often base command and default options. Can be coupled with CMD for additional options.

Example: Web application

Create a working directory with some content for a web server:

$ mkdir demo-httpd && cd demo-httpd && echo 'sample container' > index.html

Start the Dockerfile with a FROM command to indicate the base image:

$ echo 'FROM fedora:latest' >> Dockerfile

Add a RUN command to update the image and add any application and utilities:

$ echo 'RUN dnf -y update && dnf -y install httpd git  && dnf clean all' >> Dockerfile

The above example installs git. If your web content is hosted in a version control system, you can add a RUN statement to clone that data to the container. If your content is available in the build working directory, you can use the COPY command to add it to the container.

Copy to the sample index.html file into the container:

$ echo 'COPY index.html /var/www/html/index.html' >> Dockerfile

The EXPOSE line specifies that the container listens on specified network ports. It is used by the --publish-all option on the podman run command.

Document what ports are available to publish:

$ echo 'EXPOSE 80' >> Dockerfile

Specify the command to run when the container starts:

$ echo 'ENTRYPOINT /usr/sbin/httpd -DFOREGROUND' >> Dockerfile
Port bindings are not yet supported by rootless containers. If your container needs to be available on the network, build it in the root namespace. Port bindings for rootless containers is available in upstream testing for podman 1.1.0 with slirp4netns v0.3.0.

Build the image with a descriptive tag:

$ sudo podman build --tag fedora:myhttpd -f ./Dockerfile

The image will appear in the local registry:

$ sudo podman images
REPOSITORY                 TAG       IMAGE ID       CREATED         SIZE
localhost/fedora           myhttpd   223534b48a9c   3 minutes ago   474MB
docker.io/library/fedora   latest    8b38e3af7237   4 weeks ago     315MB

To make the application port available to the host device use the --publish or -p option with hostPort:containerPort numbers. An IP can also be specified as well as ranges of ports. See the man page for more options.

Run the container and publish the port:

$ sudo podman run -p 8080:80 --name myhttpd --rm fedora:myhttpd

View the port information:

$ sudo podman port myhttpd
80/tcp -> 0.0.0.0:8080

Access the web page from the host device:

$ curl localhost:8080

Access the web page from a remote location using the IP address of the host device and the published port number.

Open firewall ports, services, or sources as needed. The Fedora IoT image defaults to allowing any source on the same network through the interfaces option:

$ sudo firewall-cmd --list-all
public (active)
  target: default
  icmp-block-inversion: no
  interfaces: eth0
  sources:
  services: dhcpv6-client mdns ssh
  ports:
  protocols:
  masquerade: no
  forward-ports:
  source-ports:
  icmp-blocks:
  rich rules:

Add a port with:

$ sudo firewall-cmd --add-port 8080/tcp

More information on the firewall-cmd command can be found at firewalld.org.

Example: Interaction with GPIO interface

To interact with the GPIO interface, layer the libgpiod-utils package on the existing image or use with a container.

To layer the package:

$ sudo rpm-ostree install libgpiod-utils python3-libgpiod
$ sudo gpiodetect

To create a container for an application that works with the GPIO interface in the root namespace.

Start the Dockerfile with a FROM command to indicate the base image:

$ echo 'FROM fedora:latest' >> Dockerfile

Add a RUN command to update the image and add any application and utilities:

$ echo 'RUN dnf -y update && dnf -y install git libgpiod-utils python3-libgpiod && dnf clean all' >> Dockerfile

The fedora:latest image includes bash so we can go ahead and build the container without any specific applications to start or ports to expose. The command can be specified when we run the container.

Build the image with a descriptive tag:

$ sudo podman build --tag fedora:gpio -f ./Dockerfile

The image will appear in the localhost registry for the root namespace:

$ sudo podman images
REPOSITORY                 TAG      IMAGE ID       CREATED         SIZE
localhost/fedora           gpio     655abf78e6b9   4 minutes ago   542MB
docker.io/library/fedora   latest   8b38e3af7237   4 weeks ago     315MB

To access the host GPIO device from the container, use the --device option when you start the container:

$ sudo podman run -it --name demo-gpio --device=/dev/gpiochip0 localhost/fedora:gpio /bin/bash

Verify you can see the GPIO device:

[root@167f31750fdb /]# gpiodetect
gpiochip0 [pinctrl-bcm2835] (54 lines)

Now that the device is available from the container, continue to use the installed tools or add addition applications.

Examples for using gpioset can be found in a 2018 Fedora Magazine article: How to turn on an LED with Fedora IoT

Automate additional steps by modifying the Dockerfile and building a new container.

The images do not have to be built from a Fedora container. This Dockerfile uses a raspbian image and clones the lightshowpi project:

$ cat Dockerfile
FROM raspbian/stretch:latest
RUN apt-get -y update && apt-get -y install git-core && apt-get -y clean
WORKDIR /
RUN git clone https://togiles@bitbucket.org/togiles/lightshowpi.git && \
  cd lightshowpi && git fetch && git checkout stable

The Docker documentation includes Dockerfile best practices.

Reusing and Sharing the Containers

Once the container image is created it can be deployed to multiple devices by uploading it to a registry.

Most registries require a naming convention of the 'useraccount/description:tag' and the default for most pull commands is to look for a container with a tag of 'latest'. An image can have multiple tags and these tags are used to help identify architecture compatibility and version control.

To rename or add a tag to a local image:

$ podman tag fedora:myhttpd testuser/fedora-myhttpd:latest
$ podman tag fedora:myhttpd quay.io/testuser/fedora-myhttpd:latest

Both names will appear in the list of images but the image ID will be the same for each:

$ podman images
REPOSITORY                               TAG       IMAGE ID       CREATED        SIZE
localhost/fedora                         myhttpd   d52cbe4136e8   24 hours ago   428 MB
localhost/testuser/fedora-myhttpd        latest    d52cbe4136e8   24 hours ago   428 MB
quay.io/testuser/fedora-myhttpd          latest    d52cbe4136e8   24 hours ago   428 MB
docker.io/library/fedora                 latest    26ffec5b4a8a   4 weeks ago    283 MB

You can then push an image to a registry with podman push imageID destination.

To extract the image to a local directory in a docker format:

$ podman push quay.io/testuser/fedora-myhttpd dir:/tmp/fedora-myhttpd

For more exporting options, see the podman-push man page.

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