It's there already:

docker run --rm lambci/lambda:build-ruby2.5 /var/runtime/bin/bundler --version
Bundler version 1.17.1

I'll check why /var/runtime/bin isn't in the PATH (it might be that the ruby bootstrap modifies the PATH)

Awesome, interested to see what caused it!

Well /var/runtime/bin isn't added to PATH in Lambda – I have no idea why Amazon put bundler there.

I actually think this is a better solution than adding /var/runtime/bin to the PATH, so I'm going to reopen this.

Just pushed out a new version of build-ruby2.5 with this included – thanks for the contribution!

Thanks for the quick turnaround!