Learn the difference between copy and add instructions in dockerfile.To start with, both copy and add instructions are used to copy files from a source into the docker image.

ADD

ADD instruction copies everything from the source including the file system metadata to the destination.

Syntax

ADD <src> … <dest>

When you copy a compressed file using ADD, it automatically extracts the compressed file’s content into the destination if the compressed file is of a recognized compression format (zip, xz, bzip, gzip).

Examples

ADD http://ngrok.co/installer.file.tar.gz /ngroktemp

ADD /home/installer.zip /dockerhome/installer

COPY

COPY command copies the source to the destination in the same format the source is.

Syntax

COPY <src> … <dest>

It doesn’t work with URLs and it doesn’t extract compressed files but only copies them.

Example

COPY /home/inputs/  /dockerhome/inputs

As a final note, always prefer using COPY as it’s more transparent that ADD but at the end of the day it depends on what you want to do.