What is the Difference Between COPY and ADD Instructions in Dockerfile?

Both COPY and ADD are instructions in a Dockerfile used to copy files from the build context to a Docker image. However, there are some key differences between them:

1. Basic Functionality:

   • COPY: Simply copies local files or directories to a specified path in the target Docker image.
   • ADD: Can also copy local files or directories to the image, but it supports two additional features: first, it can handle URL source files, downloading the file specified by the URL to the image; second, it automatically handles compressed archives (such as tar archives), extracting them to the target path.

2. Usage Recommendations:

   • Since COPY focuses solely on basic copy operations, its behavior is more direct and predictable, so Docker documentation recommends using COPY when only copying files is required.
   • ADD should be used for special scenarios where COPY cannot satisfy requirements, such as when downloading network resources or automatically extracting compressed files.

3. Examples:

   • Using COPY instruction: COPY ./localfile.txt /path/in/container/localfile.txt
   • Using ADD instruction: `ADD http://example.com/examplefile.tar /path/in/container/

In summary, although ADD provides more features, it is generally recommended to use COPY in most cases to maintain the simplicity and maintainability of the Dockerfile.