What Dockerfile is and common commands

Docker can build images automatically by reading the instructions from a Dockerfile. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. Using docker build users can create an automated build that executes several command-line instructions in succession.


Order of instructions in Dockerfile matters, as all commands on Dockerfile as executed sequentially, and some instruction resulting in its own layer, increasing image size as a result. Instructions that are changing more frequently should be placed closer to the bottom, and instructions that don't change or change less frequently, should be placed higher in the Dockerfile. When one of the instruction changes, COPY for instance, all subsequent instructions must be re-run.

For more best practices see: Dockerfile best practices

Sample Dockerfile

# From Node 10, Alpine 3.9 OS.
FROM node:10-alpine3.9

# Current directory.
WORKDIR /usr/app

# Install NPM packages
COPY ./package.json ./
RUN npm install

# Copy app files
COPY ./*.js ./

# Run app
CMD ["npm", "start"]

Building image

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