Add Git branch to Jekyll configuration

[asbjornu]

This PR adds the Git branch and repository URL to the Jekyll configuration by introducing a GitMetadataProvider that encapsulate the logic that is currently in the Developer Portal's Rakefile.

[codecov]

Codecov Report

Merging #91 (16bd398) into master (a679fa7) will increase coverage by 3.30%.
The diff coverage is 91.88%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
+ Coverage   91.83%   95.13%   +3.30%     
==========================================
  Files          21       28       +7     
  Lines         612      760     +148     
==========================================
+ Hits          562      723     +161     
+ Misses         50       37      -13     

Impacted Files	Coverage Δ
.docker/entrypoint/lib/commander.rb	86.11% <66.66%> (+0.19%)	⬆️
.docker/entrypoint/lib/entrypoint.rb	90.90% <66.66%> (+29.61%)	⬆️
.docker/entrypoint/lib/git_metadata_provider.rb	72.72% <72.72%> (ø)
.docker/entrypoint/lib/gemfile_generator_exec.rb	93.93% <85.71%> (+36.36%)	⬆️
.docker/entrypoint/lib/gemfile_generator.rb	92.06% <87.50%> (+4.56%)	⬆️
.docker/entrypoint/lib/gemfile_differ.rb	98.07% <90.90%> (ø)
.docker/entrypoint/lib/argument_parser.rb	100.00% <100.00%> (ø)
.docker/entrypoint/lib/argument_string_builder.rb	100.00% <100.00%> (ø)
.docker/entrypoint/lib/arguments.rb	100.00% <100.00%> (ø)
...docker/entrypoint/lib/commands/default_commands.rb	94.11% <100.00%> (-0.89%)	⬇️
... and 16 more

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[NullableInt]

Added a few questions 😃