Note: There are better ways to do this by now. Check https://docs.docker.com/develop/develop-images/build_enhancements/#new-docker-build-secret-information for details
In order to access packages in private github repositories a Dockerfile might contain statements like this:
RUN git config --global url."https://${GITHUB_TOKEN}@github.com/".insteadOf "https://github.com/"
RUN npm install --ignore-scripts --quiet && npm cache clean --force
RUN git config --global --unset url."https://${GITHUB_TOKEN}@github.com/".insteadOf
On the CI instance this is done with docker build --build-arg GITHUB_TOKEN=${GITHUB_TOKEN} where GITHUB_TOKEN is set as environment variable.
In order for the build command to succeed on a local developer machine (aka Macbook) the GITHUB_TOKEN needs to be passed into the build command through the --build-arg command line parameter.
In order to automate this process (assuming that the github credentials are stored in the mac keychain) without having them to be stored in a permanent place the following build command can be used:
docker build --build-arg GITHUB_TOKEN=`printf 'host=github.com\\nprotocol=https\\n\\n' | git credential-osxkeychain get | tr '\\n' ' ' | sed -E 's/password=(\\w*)\\susername=(\\w*)/\\2:\\1/g'` .
What this comes down to is using the following shell command to extract the credentials from the OSX key chain and massage it into the username:password format that we need
printf 'host=github.com\\nprotocol=https\\n\\n' | git credential-osxkeychain get | tr '\\n' ' ' | sed -E 's/password=(\\w*)\\susername=(\\w*)/\\2:\\1/g'
What this does is to execute git credential-osxkeychain get, feed it with the two lines
host=github.com
protocol=https
then convert the line break from the result (the password and username from the key chain) into a space tr '\\n' ' ' (that's just because multiline sed is a pain), then extract the password and username pieces of that string and reformat it.
If the image will always be private then this is fine, but otherwise it's worth mentioning that using this method will leave your GitHub token in the Docker image history, accessible with the
docker historycommand.