Dockerfile FROM + ARG trick — parameterized dockerfile base image

For long time I thought that it was impossible to parameterize FROM instruction in a Dockerfile.

FROM node:10-alpine

Let’s say you want to reuse your Dockerfile (DRY) for multiple applications but with different versions of node (this is just one of multiple examples):

• APP_1: node:10-alpine
• APP_2: node:12-alpine

You probably ended up creating 2 Dockerfiles with different FROM instructions but basically the same content.

Enter the trick

One can parameterize the FROM instruction using an ARG instruction above!

# MAGIC! if no arg provided default to "10-alpine"
ARG node_version=10-alpine
FROM node:${node_version}
COPY package.json .
RUN npm install 
# we can use ARG for other things
ARG app_id=1
COPY packages/app_${app_id} .
CMD ["node", "app.js"]

and you can use build-args to specify:

• which version tag of node image you want to use
• even specify which folders to copy

# build app_1
app_id=1
docker build --build-arg "node_version=10-alpine" --build-arg "app_id=${app_id}" -t myapp:${app_id} .
# build app_2
app_id=2
docker build --build-arg "node_version=12-alpine" --build-arg "app_id=${app_id}" -t myapp:${app_id} .

There you have it, hope you like it!

Q. What is the first instruction in a Dockerfile?
A. FROM … not really