Save Docker Container as Image

As a full-stack developer working with Docker, you‘ve likely needed to update images frequently based on changes made in application containers at runtime. Rather than rebuilding images from scratch each time, specialized Docker commands allow containers to be "committed" to new images for ease of reuse.

In this comprehensive guide, we‘ll cover everything you need to know as a Docker expert on saving containers as images, including key concepts, step-by-step examples, use cases, customization, sharing and distribution best practices.

Docker Images vs. Containers

Before diving into workflows around committing containers as images, it‘s important to level-set on some key Docker terminology:

Docker images – Read-only templates used to build container environments. They include all dependencies and configurations needed to run the application. Images are built from a Dockerfile or pulled from registries like Docker Hub.

Docker containers – Runnable instances launched from images. Containers include a runtime environment with all the code, dependencies, settings and libraries to operate the application. Any changes made inside containers do not change the actual image.

So in summary:

• Images define what goes into a container
• Containers are changeable runtime environments built from images

This differentiation is key to understand why saving container changes as images is so useful.

Why Save Containers as Images?

In many Docker workflows, containers are launched from existing images, then developers make changes to the application environments at runtime as needed during development, testing or troubleshooting.

Some common scenarios:

• Tweaking config settings
• Adding application dependencies
• Making code changes
• Testing out database optimizations

Rather than rebuilding images from scratch via Dockerfiles each time, it‘s easier to simply "commit" the modified container to a new image. This saves the current container state as a reusable image for future use.

Some key benefits of committing container changes to new images:

• Faster rebuild times than full image builds
• Preserves current container runtime configurations
• Allows for easy image branching and versioning
• No need to redefine everything in Dockerfiles

Now let‘s walk through this container-to-image process with some examples.

How to Create a Container from an Image

First, we‘ll create a sample Nginx container from an existing image:

> docker run --name my-nginx -p 8081:80 -d nginx

Breaking this down:

• docker run = Runs a container from an image
• --name my-nginx = Names the container "my-nginx"
• -p 8081:80 = Forwards host port 8081 to container port 80
• -d = Runs detached
• nginx = Official Nginx image from Docker Hub

Verify the container is running:

> docker ps
CONTAINER ID   IMAGE     COMMAND                  PORTS                  NAMES
5842e327863b   nginx     "nginx -g ‘daemon of..."   0.0.0.0:8081->80/tcp   my-nginx

Let‘s make a simple config change inside this running Nginx container at /etc/nginx/conf.d/default.conf.

We‘ll append some custom text to the default Nginx index page output:

echo "Container version" >> default.conf

Save Container Changes as Image with Docker Commit

At this point, we have an Nginx container running from the official Nginx image, but with our own custom default config change.

To save this altered container as a new Docker image, we use the docker commit command:

> docker commit my-nginx nginx-custom

Where:

• my-nginx = Name of running container to commit
• nginx-custom = New image name

Now our container is saved as a reusable image called nginx-custom containing the runtime configuration changes.

List Docker images to confirm:

> docker images

REPOSITORY TAG     IMAGE ID   CREATED              SIZE
nginx-custom latest  7d9495d03763 About a minute ago 133MB
nginx latest  40b3f8a0b64b 2 weeks ago 133MB

We can launch new containers from this nginx-custom image to reuse our configuration:

docker run --name my-custom-nginx -p 8082:80 -d nginx-custom  

So in just a few quick commands, we took an existing container, made changes, committed to preserve state as an image, and launched a new container to persist that custom runtime configuration – all without rebuilding from a Dockerfile.

Advanced Image Configuration

When committing containers to images with docker commit, there are additional options available to customize image metadata and settings:

docker commit [OPTIONS] CONTAINER [REPOSITORY[:TAG]]

Common flags:

• -a, --author="": Author field for image
• -c, --change=[]: Apply Dockerfile instructions while committing the image
• -m, --message="": Commit message for image
• -p, --pause=true: Pause container during commit (default true)

For example, we can set the author, commit message and additional Dockerfile instructions like so:

docker commit \
  --author "John Smith" \
  --message "Changed default Nginx text" \
  --change "CMD [\"nginx\", \"-T\"]" \
  my-nginx nginx-custom

This allows for greater flexibility and customization when saving container changes as reusable images.

Compare to Rebuilding from Dockerfiles

An alternative to the docker commit approach would be to update the source Dockerfile used to initially build the image, then rebuild from scratch.

However, committing containers directly offers a few advantages:

• Faster rebuild times: No need to re-run every Dockerfile instruction from scratch
• Preserves current state: Commit picks up on any files changes, config tweaks done in running containers
• Iterates on existing images: Allows branching of images vs. linear FROM statements

In most development scenarios, docker commit provides greater convenience and flexibility for packaging incremental changes versus rebuilding full images from Dockerfiles repeatedly. Think of it like an easy way to "checkpoint" container changes back into images.

Image Tagging Best Practices

When committing containers to new images, proper image tagging is strongly recommended to keep environments organized.

Some tips:

• Use semantic versions – e.g. nginx:1.0, nginx:1.1
• Tag by features – e.g. nginx:alpine, nginx:debug
• Use dates – e.g. nginx:20220101
• Avoid :latest tag – Keep it specific

For example:

docker commit my-nginx john/nginx:1.0.1

This clearly marks the image‘s author, application name, semantic version and custom changes.

Following conventions like this keeps the purpose and environment clear for other team members down the road.

Sharing and Distributing Images

As a full-stack developer within an organization, custom images created from containers will likely need to be distributed and shared across teams and environments.

Some best practices:

• Use private Docker registries – Central hubs for storing, sharing enterprise images
• Optimize images before distribution – Smaller sizes using multi-stage builds, .dockerignore files etc.
• Implement security scanning – Check for vulnerabilities before releasing images widely
• Add documentation – Readmes on exactly what custom changes the image contains

Putting guidelines like this in place ensures the integrity, efficiency and security of container images within enterprise systems.

Docker Image Management Best Practices

As teams ramp up their usage of Docker containers and images in production, following best practices around container/image lifecycle management becomes critical:

• Container restart policies – Set restart rules for failed/exited containers
• Image cleanup cadence – Actively prune unused images to save disk space
• Tag mutation rules – Ensure proper process for modifying tags post-release
• Image monitoring – Track usage metrics per image across environments
• Validation checks – Scan for vulnerabilities, enforce conventions pre-commit

Getting control around image management and governance early on will streamline development workflows and production maintenance tremendously as reliance on container architectures grows.

Analyzing Docker Hub for Industry Trends

Reviewing public data on Docker image usage from sources like Docker Hub provides great insights into what‘s popular in production environments.

Top Images on Docker Hub

As shown above, Nginx, Alpine and Ubuntu maintain top spots for most pulled Docker images globally on Docker Hub.

This indicates developers commonly use these base OS images to build further custom containers and microservices.

We can also look at trending growth over the past year:

Fastest Growing Images on Docker Hub

Here we see Docker adoption skyrocketing in big data analytics and ML platforms like Tensorflow, Kafka and Presto.

Reviewing hub stats like this regularly spotlights what‘s emerging and popular in the container landscape.

Wrap Up

We‘ve covered a ton of ground around Docker images and containers – from key terminology and use cases, to practical examples saving containers as images with docker commit, plus best practices around customization, sharing distribution and overall image management.

Hopefully this guide has conveyed:

• The power of committing containers to reusable images
• How to easily preserve runtime state changes without dockerfiles
• Strategies and tips for organizing environments as they scale

Getting comfortable utilizing images and containers together will unlock simpler, more flexible application development workflows.