Does your Gemfile reference private Github repositories?
**Option 1: ENV variable**
```bash
export BUNDLE_GITHUB__COM=username:password
export BUNDLE_GITHUB__COM=:x-oauth-basic
```
**Option 2: bundle config**
```bash
bundle config https://github.com/bundler/bundler.git username:password
```
**Option 3: ~/.git-credentials**
```bash
echo 'https://user:[email protected]' >> ~/.git-credentials
```
**References**
https://github.com/rubygems/bundler/pull/3898
https://git-scm.com/docs/git-credential-store