git push --follow-tags uses the host machines ssh creds

[PMByrne]

Already posted this in the custom-strider project but thought it might actually get some some visibility here.

We have a build that uses the git provider.
It successfully clones our node repo, runs install, runs tests, then starts our custom script.

Our custom script run an npm prune --production, then an npm pack, we then push the new tarball into aws S3. Finally we run npm version patch with a commit message.

Everything up to this point works fine up to this point. The problem comes when we try to push the tags and the modified package.json file back into github.

+ git push --follow-tags origin master
Permission denied (publickey).
fatal: Could not read from remote repository.

If we add the id_rsa.pub file from the host machines .ssh folder as a deploy key things work fine.
Obviously this doesn't scale particularly well since we can only have that pub key as a deploy key of a single project.
How can we successfully complete this process by pushing to github?
How can we use the builds ssh creds instead of the host machines?

Our build script in case it helps (This lives in the deploy group of custom scripts):

appName=`node -e "console.log(require('./package.json').name);"`

npm version patch -m "%s (production deployment)"
npm prune --production
npm pack

# push to s3
version=`node -e "console.log(require('./package.json').version);"`
mv ./${appName}-*.tgz ./${appName}.tgz || exit 1
aws s3 cp --sse AES256 ./${appName}.tgz s3://${bucket}/deployables/${appName}-aws/${appName}-${version}.tgz || exit 1

# bump the version again for development
npm version patch -m "%s (development)"

git push --follow-tags origin master