Question 1

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 those behaviours are the reason most style guides recommend defaulting to COPY.

The first extra behaviour is archive extraction. If the source is a local tar archive in a recognised compression format, ADD unpacks it into the destination instead of copying the file. That is occasionally exactly what you want, and it is the one case where ADD genuinely earns its place. It is also a surprise if you intended to ship the archive itself.

The second is remote fetching. ADD accepts a URL and downloads it during the build. This sounds convenient but behaves poorly in practice. The download is not cached the way a local file is, the resulting layer includes whatever the server returned at build time with no integrity check, and you cannot clean up intermediate files within the same instruction. Fetching in a RUN step instead lets you verify a checksum, fail loudly on a bad download, and remove temporary files before the layer is committed.

The practical guidance is straightforward. Use COPY for everything, because it does one obvious thing and a reader does not have to check whether the source happens to be an archive. Reach for ADD only when you specifically want local tar extraction.

Both instructions participate in layer caching the same way, invalidating on file content changes. That makes ordering matter more than the choice between them: copy dependency manifests and install packages before copying application source, so that editing your code does not invalidate the expensive install layer.

Question 2

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

Question 3

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 4

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 5

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

Question 6

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 7

What is the difference between git fetch and git pull?

Accepted Answer

Fetching downloads new commits from the remote and updates your remote-tracking references. It changes nothing about your working tree or your local branches. After fetching you know what has

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