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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