00_2_Dockerfile.md

Dockerfile

Instructions for building a docker image.

Creating your Dockerfile

To take advantage of Docker layer caching (and reduce time when building the docker image), we should always have the instructions that don't often change at the top and those that change frequently, at the bottom.

FROM (base image for our application)

FROM node:lts-alpine

COPY and ADD (adding files)

All files

COPY . /app

We can ignore some files or directories with .dockerignore

.dockerignore

node_modules/

In this example, not adding the node_modules/ folder will drastically reduce the build context. However, since we are not adding node_modules/ we have to run npm install.

Some files

COPY package.json index.js /app/

If we are using WORKDIR we can use a relative path:

WORKDIR /app
COPY src .

We can also use ADD which will allow us to copy a file from a URL and/or uncompress a compressed file and add it to the directory.

ADD your-full-url/file.ext .

ADD file.zip .

To reduce times during build, we can COPY two times: one with the files that we are going to use for installing our external dependencies and the other to copy the application files. Doing this, if something changes in our source code, docker won't re-run npm install (or whatever you are using to install your dependencies), and, in consequence, it will use the cached layer.

COPY package*.json .
RUN npm install
COPY . .

RUN (running commands)

RUN npm install

ENV (setting env variables)

Single:

ENV NAME1=value

Multiple:

ENV NAME1=value1 \
    NAME2=value2

PORT (exposing the port of the container)

Remember, it exposes the port of the container NOT the host

EXPOSE 3000

USER

Buy default docker runs the application using the root user, what can be a security concern. We can create a user with scoped permissions.

In the following example we are creating a group (application) and a new system user (application) in node:alpine

RUN addgroup application && adduser -S -G application application
USER application

CMD

RUN is used at the build time (aka, when building the image) and CMD at run time (aka, when running the container).

CMD ["npm", "start"]

# You can also do the following but it will use a new shell
# CMD npm start

This requires the existence of a start script on package.json

  "scripts": {
    "start" : "node server.js"
  }

---

FROM node:alpine
RUN addgroup application && adduser -S -G application application
USER application
WORKDIR /app
RUN mkdir data
COPY . .
RUN npm install
ENV STAGE=development
EXPOSE 9090
CMD ["npm", "start"]

Then, we can build the image:

docker build -t image-name .