Copy vs add

ADD — Copy with Extras

syntax

ADD <src> <dest>

example

ADD app.tar.gz /app/
ADD https://example.com/config.json /app/config.json

Note ADD can auto-extract tar archives and fetch URLs. However, COPY is preferred for simple file copies because ADD's extra behavior can be surprising. Using ADD for URL downloads also defeats layer caching — use RUN curl instead for better control.

Using ADD When You Mean COPY

DK · Common Mistakes

syntax

# ADD has hidden behaviors:
ADD app.tar.gz /app/    # auto-extracts
ADD https://x.com/f /f  # downloads

example

# Use COPY for plain file copies (clear intent):
COPY package.json ./
COPY src/ ./src/

# Only use ADD when you specifically want extraction:
ADD assets.tar.gz /app/assets/

Note ADD auto-extracts tar archives and can fetch URLs, which is rarely what you want. COPY does exactly one thing — copies files. Using COPY makes your Dockerfile easier to reason about and avoids accidental extraction of files you wanted to keep as archives.

DKDocker