In the docker docs getting started tutorial part 2, it has one make a Dockerfile. It instructs to add the following lines:
# Set the working directory to /app WORKDIR /app # Copy the current directory contents into the container at /app COPY . /app What is /app, and why is this a necessary step?
There are two important directories when building a docker image:
WORKDIR directory.It's the directory on the host machine where docker will get the files to build the image. It is passed to the docker build command as the last argument. (Instead of a PATH on the host machine it can be a URL). Simple example:
docker build -t myimage . Here the current dir (.) is the build context dir. In this case, docker build will use Dockerfile located in that dir. All files from that dir will be visible to docker build.
The build context dir is not necessarily where the Dockerfile is located. Dockerfile location defaults to current dir and is otherwise indicated by the -f otpion. Example:
docker build -t myimage -f ./rest-adapter/docker/Dockerfile ./rest-adapter Here build context dir is ./rest-adapter, a subdirectory of where you call docker build; the Dokerfile location is indicated by -f.
It's a directory inside your container image that can be set with the WORKDIR instruction in the Dockerfile. It is optional (default is /, but the parent image might have set it), but setting it is considered a good practice. Subsequent instructions in the Dockerfile, such as RUN, CMD and ENTRYPOINT will operate in this dir. As for COPY and ADD, they use both...
These two commands have <src> and <dest>.
<src> is relative to the build context directory.<dest> is relative to the WORKDIR directory.For example, if your Dockerfile contains...
WORKDIR /myapp COPY . . ADD data/mydata.csv /usr/share/mydata.csv then...
COPY command will copy the contents of your build context directory to the /myapp dir inside your docker image.ADD command will copy file data/mydata.csv under your build context dir to the /usr/share dir at the docker image filesystem root level (<dest> in this case starts with /, so it's an absolute path).
--from=<stage> option on commands like COPY. These change the "build context" to be the output from the specified stage of the build rather than the native filesystem.WORKDIR is a good practice because you can set a directory as the main directory, then you can work on it using COPY, ENTRYPOINT, CMD commands, because them will execute pointing to this PATH.
Docker documentation: https://docs.docker.com/engine/reference/builder/
The WORKDIR instruction sets the working directory for any RUN, CMD, ENTRYPOINT, COPY and ADD instructions that follow it in the Dockerfile. If the WORKDIR doesn’t exist, it will be created even if it’s not used in any subsequent Dockerfile instruction.
The WORKDIR instruction can be used multiple times in a Dockerfile. If a relative path is provided, it will be relative to the path of the previous WORKDIR instruction.
Dockerfile Example:
FROM node:alpine WORKDIR '/app' COPY ./package.json ./ RUN npm install COPY . . CMD ["npm", "run", "start"] A alpine node.js was created and the workdir is /app, then al files are copied them into /app
Finally npm run start command is running into /app folder inside the container.
You should exec the following command in the case you have sh or bash tty:
docker exec -it <container-id> sh
or
docker exec -it <container-id> bash
After that you can do ls command and you will can see the WORKDIR folder.
I hope it may help you
root@c95328d92adf:/app# pwd and the output: /app Does this mean that /app is just a sub-directory of the a root directory in the container?You need to declare a working directory and move your code into it, because your code has to live somewhere. Otherwise your code wouldn't be present and your app wouldn't run. Then when commands like RUN, CMD, ENTRYPOINT, COPY, and ADD are used, they are executed in the context of WORKDIR.
/app is an arbitrary choice of working directory. You could use anything you like (foo, bar, or baz), but app is nice since it's self-descriptive and commonly used.
COPY . /app, you findCOPY . .