COPY failed: stat /var/lib/docker/tmp/docker-xxx : no such file or directory

Question

I have a github actions workflow to build a docker image:

name: Backend-Demo Docker Image CI
on:
  push:
    branches: [ master ]
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - name: Login to Azure Container Registry
        run: echo ${{ secrets.REGISTRY_PASSWORD }} | docker login ${{ secrets.LOGIN_SERVER_URL }} -u ${{ secrets.REGISTRY_USERNAME }} --password-stdin
      - name: Get the version
        id: vars
        run: echo ::set-output name=tag::$(echo ${GITHUB_REF:10})
      - name: Build the tagged Docker image
        run: docker build . --file backend/Dockerfile --tag backend-demo/spring-boot:v1.0

The Dockerfile is:

FROM openjdk:14-alpine
MAINTAINER example.com
RUN mkdir -p /opt/demo-0.0.1/lib
# Setting application source code working directory
WORKDIR /opt/demo-0.0.1/
COPY target/demo-0.0.1-SNAPSHOT.jar /opt/demo-0.0.1/lib/demo-0.0.1-SNAPSHOT.jar
# ADD target/demo-0.0.1-SNAPSHOT.jar /opt/demo-0.0.1/lib/

RUN sh -c 'touch demo-0.0.1-SNAPSHOT.jar'
ENTRYPOINT ["java"]
CMD ["-jar", "/opt/demo-0.0.1/lib/demo-0.0.1-SNAPSHOT.jar"]

But when I execute this workflow I got this error at the COPY instruction:

Step 5/8 : COPY target/demo-0.0.1-SNAPSHOT.jar /opt/demo-0.0.1/lib/demo-0.0.1-SNAPSHOT.jar
COPY failed: stat /var/lib/docker/tmp/docker-builder851513197/target/demo-0.0.1-SNAPSHOT.jar: no such file or directory
##[error]Process completed with exit code 1.

I have been checking and it looks a typical error when the file we have the Dockerfile in a different directory like my instruction:

docker build . --file backend/Dockerfile --tag backend-demo/spring-boot:v1.0

I also don't have .dockerignore file and my Dockerfile is called Dockerfile precisely.

The target/demo-0.0.1-SNAPSHOT.jar file I am trying to copy is present in my github repository Not sure what could be happening with the context, but probably this answer could be a good hint?

Answer 1

When you run

docker build . --file backend/Dockerfile ...

The path argument . becomes the context directory. (Docker actually sends itself a copy of this directory tree, which is where the /var/lib/docker/tmp/... path comes from.) The source arguments of COPY and ADD instructions are relative to the context directory, not relative to the Dockerfile.

If your source tree looks like

.
+-- backend
| \-- Dockerfile
\-- target
  \-- demo-0.0.1-SNAPSHOT.jar

that matches the Dockerfile you show. But if instead you have

.
+-- backend
  +-- Dockerfile
  \-- target
    \-- demo-0.0.1-SNAPSHOT.jar

you'll get the error you see.

If you don't need to refer to anything outside of the context directory, you can just change what directory you're passing to docker build

COPY target/demo-0.0.1-SNAPSHOT.jar /opt/demo-0.0.1/lib/demo-0.0.1-SNAPSHOT.jar

docker build backend ...

Or, if you do have other content you need to copy in, you need to change the COPY paths to be relative to the topmost directory.

COPY backend/target/demo-0.0.1-SNAPSHOT.jar /opt/demo-0.0.1/lib/demo-0.0.1-SNAPSHOT.jar
COPY common/config/demo.yml /opt/demo-0.0.1/etc/demo.yml

docker build . -f backend/Dockerfile ...