Hello!
When I’m building Docker images, sometimes I need to pass data from the build agent (e.g. my CI pipeline) into the build process. Often, I also want to echo that data into the logs so I can use it for troubleshooting or validation later. Docker supports this!
These examples were all tested in Docker for Mac:
docker --version Docker version 19.03.13, build 4484c46d9d
First, declare your build-time data as an ARG in your Dockerfile:
FROM alpine:3.7 ARG USEFUL_INFORMATION ENV USEFUL_INFORMATION=$USEFUL_INFORMATION RUN echo "Useful information: $USEFUL_INFORMATION"
In this example, I’ve also set an ENV variable so I can RUN a command to print out the new ARG.
Now, just build like ususal:
docker build --tag test_build_args --build-arg USEFUL_INFORMATION=1337 . Sending build context to Docker daemon 10.24kB Step 1/4 : FROM alpine:3.7 ---> 6d1ef012b567 Step 2/4 : ARG USEFUL_INFORMATION ---> Using cache ---> 18d20c437445 Step 3/4 : ENV USEFUL_INFORMATION=$USEFUL_INFORMATION ---> Using cache ---> b8bbdd03a1d1 Step 4/4 : RUN echo "Useful information: $USEFUL_INFORMATION" ---> Running in a2161bfb75cd Useful information: 1337 Removing intermediate container a2161bfb75cd ---> 9ca56256cc19 Successfully built 9ca56256cc19 Successfully tagged test_build_args:latest
If you don’t pass in a value for the new ARG, it resolves to an empty string:
docker build --tag test_build_args . Sending build context to Docker daemon 10.24kB Step 1/4 : FROM alpine:3.7 ---> 6d1ef012b567 Step 2/4 : ARG USEFUL_INFORMATION ---> Using cache ---> 18d20c437445 Step 3/4 : ENV USEFUL_INFORMATION=$USEFUL_INFORMATION ---> Running in 63e4b0ce1fb7 Removing intermediate container 63e4b0ce1fb7 ---> 919769a93b7d Step 4/4 : RUN echo "Useful information: $USEFUL_INFORMATION" ---> Running in 73e158d1bfa6 Useful information: Removing intermediate container 73e158d1bfa6 ---> f928fc025270 Successfully built f928fc025270 Successfully tagged test_build_args:latest
Some details:
- Don’t use this for secrets.
- None of the arguments I’ve needed to pass in had sane defaults. If yours do, Docker supports them.
That’s it! Happy building,
Adam
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