Dockerfile: ADD vs COPY

COPY and ADD are both Dockerfile instructions that serve similar purposes. They let you copy files from a specific location into a Docker image.

COPY

The COPY instruction copies new files or directories from <src> and adds them to the filesystem of the container at the path <dest>.

COPY has two forms:

COPY [--chown=<user>:<group>] <src>... <dest>
COPY [--chown=<user>:<group>] ["<src>",... "<dest>"] (this form is required for paths containing whitespace)

ADD

ADD has two forms:

ADD [--chown=<user>:<group>] <src>... <dest>
ADD [--chown=<user>:<group>] ["<src>",... "<dest>"] (this form is required for paths containing whitespace)

Dockerfile best practice for copying from a URL

Docker suggests that it is often not efficient to copy from a URL using ADD, and it is best practice to use other strategies to include the required remote files.

COPY only supports the basic copying of local files into the container, while ADD has some features (like local-only tar extraction and remote URL support) that are not immediately obvious. Consequently, the best use for ADD is local tar file auto-extraction into the image, as in ADD rootfs.tar.xz /.

— Dockerfile Best Practices
Because image size matters, using ADD to fetch packages from remote URLs is strongly discouraged; you should use curl or wget instead. That way you can delete the files you no longer need after they’ve been extracted and you don’t have to add another layer in your image. For example, you should avoid doing things like:

ADD http://example.com/big.tar.xz /usr/src/things/
RUN tar -xJf /usr/src/things/big.tar.xz -C /usr/src/things
RUN make -C /usr/src/things all

And instead, do something like:

RUN mkdir -p /usr/src/things \
    && curl -SL http://example.com/big.tar.xz \
    | tar -xJC /usr/src/things \
    && make -C /usr/src/things all

For other items (files, directories) that do not require ADD’s tar auto-extraction capability, you should always use COPY.

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