Container image not buildable

[Nosmoht]

Hi all,

i just checked out the code and tried to build the container image using

$ docker build -t doorkeeper .

First issue i ran into:

#8 0.596 Warning: the running version of Bundler (2.0.1) is older than the version that created the lockfile (2.2.21). We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.

After changing Dockerfile from

RUN gem install bundler:2.0.1

to

RUN gem install bundler:2.2.21

i could get rig of the warning but ran into the next issue:

#10 0.525 Your Ruby version is 2.6.5, but your Gemfile specified 3.1.0

So i finally change Dockerfile from

FROM ruby:2.6.5

to

FROM ruby:3.1.0

to get a successful build.

If you agree i'll send a PR to fix this.

Kind regards

[hagaivcita]

quite needed since the dockerfile is outdated (both ruby and bundler are not inline with Gemfile)