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.