docker · Docker
The message
COPY failed: file not found in build context or excluded by .dockerignore
What it means
COPY can only take files from inside the build context, and the file is not there. The context is the last argument to docker build, so usually you pointed at a parent folder with ../x, or .dockerignore filtered the file out — classic when node_modules or *.env is ignored and then something inside it is copied. The message names both causes, so read cat .dockerignore first and then rewrite the path relative to the context root. Widening the context to fix it means the whole folder gets sent to the daemon, which makes every build slower.
The fix
cat .dockerignore- Printed by
- docker
- Docker
- 12
Docker errors only become readable once you place them in a layer — the client failing to reach the daemon, the registry refusing you, a RUN failing during the build, and a container dying the instant it starts are four different problems — and for the build and run layers the line itself is not the reason: the reason is in the output of the command that was running inside, while the cost of each fix differs by layer too.
Reading an error message
- Read from the first line down. The lower you go the more it is about the tool’s internals; the cause is usually at the top.
- If there is a file and a line number, start there — not the top stack frame, but the topmost line that names a file you wrote.
- Search the message verbatim, but strip your own paths and variable names first; those are what stop the search from matching.
- The same condition is worded differently across tool versions. If results look wrong, add the version number to the query.
- Before pasting a fix, check what it throws away. Some of these cannot be undone.
Common questions
Q. What does “COPY failed: file not found in build context or excluded by .dockerignore” mean?
COPY can only take files from inside the build context, and the file is not there. The context is the last argument to docker build, so usually you pointed at a parent folder with ../x, or .dockerignore filtered the file out — classic when node_modules or *.env is ignored and then something inside it is copied. The message names both causes, so read cat .dockerignore first and then rewrite the path relative to the context root. Widening the context to fix it means the whole folder gets sent to the daemon, which makes every build slower.
Q. How do I fix it?
cat .dockerignore — before running it, check the explanation above for what this command discards.
Q. Which tool prints this?
docker. It sits under Docker, and the message runs to 12 words.