docker -v no more needed? and Dockerfile - docker

Question

I've read tutorials about use docker:
docker run -it -p 9001:3000 -v $(pwd):/app simple-node-docker
but if i use:
docker run -it -p 9001:3000 simple-node-docker
it's working too? -v is not more needed? or is taking from the Dockerfile the line WORKDIR?
FROM node:9-slim
# WORKDIR specifies the directory our
# application's code will live within
WORKDIR /app
another tutorials use mkdir ./app on the workfile, anothers don't, so WORKDIR is enough to docker create the folder automatically if does not exist

Answer 1

There are two common ways to get application content into a Docker container. Many Node tutorials I've seen confusingly do both of them. You don't need docker run -v, provided you docker build your container when you make changes.
The first way is to copy a static copy of the application into the image. You'd do this via a Dockerfile, typically looking something like this:
FROM node
WORKDIR /app
# Install only dependencies now, to make rebuilds faster
COPY package.json yarn.lock ./
RUN yarn install
# NB: node_modules is in .dockerignore so this doesn't overwrite
# the previous step
COPY . ./
RUN yarn build
CMD ["yarn", "start"]
The resulting Docker image is self-contained: if you have just the image (maybe you docker pulled it from a repository) you can run it, as you note, without any special -v option. This path has the downside that you need to re-run docker build to recreate the image if you've made any changes.
The second way is to use docker run -v to inject the current source directory into the container. For example:
docker run \
--rm \ # clean up after we're done
-p 3000:3000 \ # publish a port
-v $PWD:/app \ # mount current directory over /app
-w /app \ # set default working directory
node \ # image to run
yarn start # command to run
This path hides everything in the /app directory in the image and replaces it in the container with whatever you have in your current directory. This requires you to have a built functional copy of the application's source tree available, and so it supports things like live reloading; helpful for development, not what you want for Docker in production.
Like I say, I've seen a lot of tutorials do both things:
# First build an image, populating /app in that image
docker build -t myimage .
# Now run it, hiding whatever was in /app
docker run --rm -p3000:3000 -v$PWD:/app myimage
You don't need the -v option, but you do need to manually rebuild things if your application changes.
$EDITOR src/file.js
yarn test
sudo docker build -t myimage .
sudo docker run --rm -p3000:3000 myimage
As I note here the docker commands require root-equivalent permission; but on the flip side the final docker run command is very close to what you'd run "for real" (maybe via Docker Compose or Kubernetes, but without requiring a copy of the application source).