COPY from_context_path to_container_pathAlthough the Dockerfile is run and built on a given context path to build an image, no files are mirrored to the corresponding Docker Container environment.
From the above example, when we run npm install inside the container, the package.json does not exist inside the container. So the npm install will fail.
Therefore, before the npm install, we can use the COPY instruction to copy all the files from the relative context path (first argument) to the container path (second argument).
FROM node:alpine
WORKDIR /usr/app
COPY ./ ./
RUN npm install
CMD ["npm", "start"]Minimizing Cache Busting
If you know that Docker can cache build processes, you will notice that COPY ./ ./ is not a plausible way to copy files into the container. This is because every time you change a file or even a single character in your project, the instructions after COPY ./ ./ will be rebuilt without caching. Therefore, the npm install will drag the whole build process every time you build.
FROM node:alpine
WORKDIR /usr/app
COPY ./package.json ./
RUN npm install
COPY ./ ./
CMD ["npm", "start"]
A preferred way to copy files into the container is to first copy the package.json or other package management files in different languages and run the dependency install.
This ensures that npm install is only rebuilt when there are changes in package.json and saves us a lot of time.
References