Difference between ADD and COPY in Dockerfile

In a Dockerfile, both ADD and COPY are used to copy files or directories from the source on the host machine to the image. 

However, they have some key differences:

Basic Functionality:

• COPY: A straightforward command for copying files or directories from the source (host) into the container image.
• ADD: Does everything that COPY does but with additional functionalities.

Additional Features in ADD:

• Automatic Extraction of Archives: ADD can automatically unpack compressed files (e.g., .tar, .tar.gz, .tar.bz2) into the container. This feature is not available with COPY.
• Remote URL Support: ADD allows you to specify a URL as the source, downloading the file directly into the container. COPY only supports local files and directories.

When to Use:

• COPY is generally preferred for simple file or directory copying tasks because it’s more explicit and doesn't introduce unintended behaviors like auto-extraction or downloading.
• ADD is more appropriate when you specifically need to extract a local archive or download a file from a remote location.

Example

Dockerfile:
# Using COPY (ideal for regular file/directory copy)
COPY ./localfile.txt /app/

# Using ADD for a compressed file
ADD ./localfile.tar.gz /app/

# Using ADD to download a file from a URL
ADD http://example.com/file.tar.gz /app/

Best Practice: 

To avoid potential side effects, use COPY unless you need one of the additional features provided by ADD.