Build/Modify a Docker/Podman image

Docker

Set up Docker and workshop files

A Dockerfile is a recipe for how to build a docker image. The best way to learn to write Dockerfiles is to start off with one that is already written and modify it for your needs.

Open up the Dockerfile in the activity-files folder.

You’ll notice we have this at the top for you:

FROM cansav09/practice-image:1

This means we’re going to take the existing image called, cansav09/practice-image:1 and build on to it. This image will be our base. There are so many Docker images out there, that it might be that someone has already created a docker image with most of the functionality you need for your project.

Step 1: Use any file editor to open up the Dockerfile

Usually double clicking will work.

Step 2: Inspect the Dockerfile

Double click it and open the file to take a look at it with any default text editor.

Step 3: Change your working directory to activity-files

Now in your Command Prompt or Terminal navigate to the activity-files folder. Use cd and don’t forget to use tabs so you don’t have to spell everything exactly.

Step 4: Build the image from the Dockerfile

With activity-files as your working directory, run the following:

docker build . -t cool-new-image

Optionally you could call this from somewhere else and use the -f option to specify the file path to the Dockerfile. But in the scenario above it just grabs the Dockerfile in our working directory.

Step 4: Inspect new image!

Let’s see if we have an image!

docker image ls

Step 5: Run the new image

Navigate back to your Docker desktop and the images window or run docker ps or podman ps. If your image built successfully, you should see a new image in your list!
Let’s try running that image.

docker run cool-new-image

Step 6: See the minor difference!

We should have a message: Yay! I built a Docker image pop up upon building the image. Not super useful but we can see how we’ve edited the original image.

Step 7: Edit the Dockerfile so it has the installation step for rmarkdown package and remove the CMD step

For anything we need ran in this image we are building we need to use the RUN command followed by the installation steps we’d need.

Open up the file called Dockerfile in activity-files.
Copy and paste this into your Dockerfile below where it says # Add a new package here so we can add the rmarkdown package.

RUN Rscript -e  "options(warn = 2);install.packages('rmarkdown', \
    repos = 'https://cloud.r-project.org/')"

AND remove the CMD line.
Save your edited Dockerfile.

Step 8: Re-build now that we’ve edited the Dockerfile

Now re-run docker build (or podman build) as you did in the previous section. This time we’ll add a versioning tag using : in the -t option.

docker build . -t cool-new-image:2

If all built successfully, you should see a message like:

=> exporting to image                                                     0.0s
=> => exporting layers                                                    0.0s
=> => writing image sha256:ayuahgfuiseohfauwheufhauwihefuahweufhawfbuibe  0.0s
=> => naming to docker.io/library/cool-new-image:2

Step 9: Run container from cool-new-image:2

Now let’s retry running the script from here but we will need to specify the volume again!

First run the container using the 2 image:

docker run -v $PWD:/home cool-new-image:2

Step 10: Re-Retry calling the script

Run docker ps or podman ps can get you the container ID. Or look on your Docker Desktop. Try running the script using the following command:

docker exec -it <REPLACE_WITH_CONTAINER_ID> bash /home/run_analysis.sh

Podman