Dockerfile中ADD与COPY命令失效问题求助

Let’s break down the common issues that cause files copied via COPY/ADD to disappear in Heroku’s container environment, plus actionable fixes to resolve them:

• Set your working directory explicitly
  Heroku defaults to /app as the container’s working directory, but declaring this at the top of your Dockerfile eliminates relative path confusion. Add this line before any COPY/ADD commands:

  WORKDIR /app
  

  Now all subsequent copy commands will target /app by default. For example, COPY . . copies everything from your local project root into /app—exactly the directory you land in when running heroku run bash.

• Audit your .dockerignore file
  This is one of the most overlooked culprits. If your .dockerignore lists the files/directories you’re trying to copy (e.g., luup/, config/, or even a misplaced .), Docker will skip copying them entirely. Open your .dockerignore and remove any lines that exclude the content you need in the container.

• Validate your COPY syntax with the working directory
  With WORKDIR /app set, use clean, reliable syntax:

  • Copy your entire local project to /app: COPY . .
  • Copy the local luup directory to /app/luup: COPY luup ./luup/
  • Copy requirements.txt directly to /app: COPY requirements.txt .
    Stick to relative paths here—absolute paths are unnecessary and can introduce unexpected behavior.

• Test locally first to rule out Heroku-specific issues
  Before deploying, build and run your image locally to confirm files are present:

  # Build the image
  docker build -t my-heroku-app .
  # Run a temporary container and open bash
  docker run -it --rm my-heroku-app bash
  

  If files are missing locally, the problem is in your Dockerfile or build context—not Heroku. If they’re present locally but missing on Heroku, double-check that you’re pushing the correct image tag to Heroku.

• Check Heroku’s build logs
  When you run heroku container:push web, scan the logs for lines related to COPY commands. You should see entries like:

  Step 5/10 : COPY . .
  ---> Using cache
  ---> abc123def456
  

  If these lines are missing or show errors (e.g., "no such file or directory"), your files aren’t available in the build context when Heroku processes your image.

• Don’t overwrite /app later in your Dockerfile
  Ensure you don’t have commands that delete or replace the /app directory after copying files. For example, a RUN rm -rf /app or VOLUME /app command would wipe out your copied content.

If you’ve tried all these steps and still have issues, sharing your full Dockerfile and .dockerignore content can help pinpoint the problem.

内容的提问来源于stack exchange，提问作者Jay Jung