Question 1

What is the difference between a Docker image and a container?

Accepted Answer

An image is a static, read-only template. A container is a running, or stopped, instance created from one. The relationship is close to that between a program on disk and a process running it: one image can produce any number of containers, each with its own writable layer and its own lifecycle.

Images are built in layers, each capturing the filesystem changes from one build instruction. Layers are shared between images, which is why pulling a second image based on the same foundation downloads far less than the first. They are also immutable, which is what makes builds cacheable and distribution efficient.

When a container starts, a thin writable layer is added on top. Everything the process writes goes there, leaving the image untouched. That is why two containers from one image do not see each other files, and why removing a container discards those changes unless they were written to a mounted volume.

Several practical consequences follow.

- Data written inside a container is ephemeral by default. Anything that must outlive the container needs a volume or bind mount.
- Stopping a container does not delete it. A stopped container still holds its writable layer and still occupies disk, which is why stopped containers accumulate and why listing all containers rather than only running ones is often surprising.
- Changing a running container does not change the image. To make a change permanent you rebuild, which is the basis of reproducible deployments.

The terminology overlaps in casual use, and people often say image when they mean container or the reverse. Keeping them distinct matters most when reasoning about state, because almost every question about where data went resolves to understanding which of the two is holding it.

Question 2

What is the difference between a Docker volume and a bind mount?

Accepted Answer

Both make data survive a container being removed, and they differ in who manages the storage. A bind mount maps a specific path on the host into the container. You control exactly where the data

Question 3

Why is my Docker image so large and how do I shrink it?

Accepted Answer

Image size is the sum of every layer, and layers are immutable. That second point catches people out: deleting a file in a later instruction does not reclaim the space, because the earlier layer

Question 4

Why does my Docker container exit immediately after starting?

Accepted Answer

A container lives exactly as long as its main process. When the process started by the image's CMD or ENTRYPOINT finishes, the container stops, regardless of anything else running inside it. So an

Question 5

What is the difference between COPY and ADD in a Dockerfile?

Accepted Answer

Both instructions move files from the build context into the image, and for ordinary files they behave identically. The difference is that ADD has two extra behaviours that trigger automatically, and

Question 6

Why is my Docker build so slow?

Accepted Answer

Most slow builds are slow because the cache is being invalidated unnecessarily, not because the work itself is expensive. Each instruction produces a layer, and once one is invalidated every

Question 7

What is a build artifact, and why does reproducibility matter?

Accepted Answer

An artifact is the packaged output of a build — a compiled binary, a container image, a library archive — and reproducibility means that building the same source produces an identical artifact, every

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