Docker build: bundler version mismatch for bountysource service

# Setup
- Docker Host: Ubuntu 20.04
- Docker Version: Docker version 19.03.13, build 4484c46d9d
- Bountysource revision: 0fd673adb4fa8eddeeec230c4e4751f855ddbadf

# Reproduction

Clone bountysource-core

Modify the env file as needed

```
$ docker-compose build
pgsql uses an image, skipping
sphinx uses an image, skipping
elasticsearch uses an image, skipping
Building bountysource
Step 1/9 : FROM ruby:2.7.0
---> ea1d77821a3c
Step 2/9 : RUN apt-get update -qq && apt-get install -y nodejs postgresql-client
---> Using cache
---> d2306b01ece2
Step 3/9 : RUN mkdir -p /app
---> Using cache
---> bfcc592d70b2
Step 4/9 : WORKDIR /app
---> Using cache
---> f1ba3862ab98
Step 5/9 : COPY Gemfile Gemfile.lock ./
---> Using cache
---> eb02aae40ef3
Step 6/9 : RUN bundle install
---> Running in af179e6a42ed
Warning: the running version of Bundler (2.1.2) is older than the version that created the lockfile (2.1.4). We suggest you to upgrade to the version that created the lockfile by running `gem install bundler:2.1.4`.
Your Ruby version is 2.7.0, but your Gemfile specified 2.7.1
ERROR: Service 'bountysource' failed to build: The command '/bin/sh -c bundle install' returned a non-zero code: 18
```
Seems to be this is an issue in one of the ~docker-compose files~ _Gemfiles_.