https://github.com/bundler/bundler/pull/6558#issuecomment-394210911

bundle_env_in_docker.sh

$ ls .
Dockerfile
$ cat Dockerfile
FROM ruby:2.5.1

RUN bundle init
RUN echo "gem 'bundler_diffgems'" >> Gemfile
RUN bundle install --path vendor/bundle
CMD ["/bin/bash"]
$ docker build .
Sending build context to Docker daemon  49.15kB
Step 1/5 : FROM ruby:2.5.1
 ---> 677dc4db9b8f
Step 2/5 : RUN bundle init
 ---> Using cache
 ---> b80c145acbaa
Step 3/5 : RUN echo "gem 'bundler_diffgems'" >> Gemfile
 ---> Using cache
 ---> 54b57d506340
Step 4/5 : RUN bundle install --path vendor/bundle
 ---> Using cache
 ---> f2ab105b7f84
Step 5/5 : CMD ["/bin/bash"]
 ---> Using cache
 ---> c51d2fa0b9ff
Successfully built c51d2fa0b9ff
$ docker run -it c51d2fa0b9ff bundle env
fatal: Not a git repository (or any of the parent directories): .git
## Environment

```
Bundler       1.16.2
  Platforms   ruby, x86_64-linux
Ruby          2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
  Full Path   /usr/local/bin/ruby
  Config Dir  /usr/local/etc
RubyGems      2.7.7
  Gem Home    /usr/local/bundle
  Gem Path    /root/.gem/ruby/2.5.0:/usr/local/lib/ruby/gems/2.5.0:/usr/local/bundle
  User Path   /root/.gem/ruby/2.5.0
  Bin Dir     /usr/local/bundle/bin
Tools
  Git         2.11.0
  RVM         not installed
  rbenv       not installed
  chruby      not installed
```

## Bundler Build Metadata

```
Built At          2018-06-18
Git SHA
Released Version  false
```

## Bundler settings

```
path
  Set for your local app (/usr/local/bundle/config): "vendor/bundle"
  Set via BUNDLE_PATH: "/usr/local/bundle"
silence_root_warning
  Set via BUNDLE_SILENCE_ROOT_WARNING: true
app_config
  Set via BUNDLE_APP_CONFIG: "/usr/local/bundle"
```

## Gemfile

### Gemfile

```ruby
# frozen_string_literal: true

source "https://rubygems.org"

git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }

# gem "rails"
gem 'bundler_diffgems'
```

### Gemfile.lock

```
GEM
  remote: https://rubygems.org/
  specs:
    addressable (2.5.2)
      public_suffix (>= 2.0.2, < 4.0)
    bundler_diffgems (0.3.0)
      gems_comparator
      parallel
    faraday (0.15.2)
      multipart-post (>= 1.2, < 3)
    gems_comparator (0.2.0)
      octokit
    multipart-post (2.0.0)
    octokit (4.9.0)
      sawyer (~> 0.8.0, >= 0.5.3)
    parallel (1.12.1)
    public_suffix (3.0.2)
    sawyer (0.8.1)
      addressable (>= 2.3.5, < 2.6)
      faraday (~> 0.8, < 1.0)

PLATFORMS
  ruby

DEPENDENCIES
  bundler_diffgems

BUNDLED WITH
   1.16.2
```