How to fix ADD failed: file not found in build context or excluded by .dockerignore in Docker

First, confirm that the file you're trying to ADD actually exists within your build context. If you're running:

docker build -t myapp .

Then the build context is the current directory (.). List all files to verify:

ls -la

If the file is in a parent directory or elsewhere, you have two options:
1. Move the file into the build context
2. Change the build context to include the file (see next steps)

The .dockerignore file (if it exists) may be excluding your file. Check for patterns that match your file:

cat .dockerignore

Common problematic patterns:
- * (excludes everything)
- *.log (might match your file extension)
- Directory patterns that include your file

To fix, either:
1. Remove the pattern excluding your file
2. Add an exception using ! before the pattern:

# Exclude all logs
*.log
# But include this specific one
!important.log

Note: The last matching line wins, so order matters!

If your file is outside the current directory, change the build context. The syntax is:

docker build -t imageName -f /path/to/Dockerfile /path/to/context

Example: If your Dockerfile is in ./docker/ but needs to COPY files from the project root:

# Instead of this (fails):
docker build -t myapp ./docker

# Use this (context is parent directory):
docker build -t myapp -f ./docker/Dockerfile .

For docker-compose, adjust the context in your docker-compose.yml:

services:
  app:
    build:
      context: ../..  # Go up two directories
      dockerfile: ./project/docker/Dockerfile

Ensure your ADD/COPY paths are relative to the build context, not the Dockerfile location. You cannot use ../ to go outside the context:

# ❌ WRONG - tries to go outside context
ADD ../config.json /app/

# ✅ CORRECT - relative to context root
ADD config.json /app/

# ✅ CORRECT - file in subdirectory of context
ADD configs/app.json /app/

Also check for trailing comments (they're not ignored!):

# ❌ WRONG - Docker looks for "file.txt # comment"
COPY file.txt /app/ # important file

# ✅ CORRECT
# important file
COPY file.txt /app/

Ensure Docker can read the file:

# Check file permissions
ls -l yourfile.txt

# Make readable if needed
chmod 644 yourfile.txt

Watch for case sensitivity issues, especially when building on case-insensitive filesystems (macOS/Windows) that deploy to Linux:

# If your file is "Config.json" but you write:
COPY config.json /app/  # This might fail on Linux!

Always match the exact case.