Master Docker Commit: From Build to Image – Deep Dive into Container Mechanics

docker commit's role and its relationship with build

Docker build creates an image from a Dockerfile, generating a new layer for each instruction. The RUN command creates a single‑layer image via a docker commit operation, as illustrated in the diagram below.

Build considerations from the perspective of docker commit

The docker commit command packages the writable layer of a running container into a new image. When a RUN instruction executes, Docker follows these steps:

Execute the command inside a temporary container.

All filesystem changes are recorded in the container's read‑write layer.

The Docker daemon checks the command's exit status:

If the status is 0, the daemon packs the read‑write layer into a new image.

If the status is non‑zero, the build fails and the layer is not created.

If the command never returns, the build hangs indefinitely.

Two key concepts must be understood: the container and the return status . Early Docker versions lacked resource limits for docker build, making it unsuitable for production. Newer releases add some limits, but they still do not match the isolation provided by docker run, and they lack fine‑grained capability management.

docker commit's underlying principle

According to docker help commit, the command "Create a new image from a container's changes" . The process involves three concepts: Image, Container, and Changes.

The Docker daemon runs a container from an image, providing a layered filesystem with a topmost read‑write layer.

All updates to the filesystem are written to this read‑write layer.

When docker commit is invoked, Docker packages only the read‑write layer into a new image, preserving image independence.

The commit operation does not include data stored in volumes, host configuration files, or virtual filesystems such as /proc and /sys. For example, a MySQL container’s database files residing in a volume are not captured by docker commit.

Conclusion

Understanding Docker deeply starts with the docker build command; docker commit serves as the practical implementation that bridges Dockerfile, containers, and final images, encompassing most core Docker concepts.