bump ruby version in template dockerfile

[themoah]

bumping ruby & alpine version in dockerfile

Description

ruby 2.7 was released recently
alpine 3.7 reached its EOL.

Motivation and Context

Which issue(s) this PR fixes

How Has This Been Tested?

manually build (before and after the change).
docker run -p 8080:8080 ruby-template:0.1
and then
curl localhost:8080
which returned
Hello world from the Ruby template (as expected)
docker log:

2020/01/05 13:33:25 Forking fprocess.
2020/01/05 13:33:25 Wrote 35 Bytes - Duration: 0.072239 seconds

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Version change (see: Impact to existing users)

Impact to existing users

newer version with backward compatibility.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[derek]

Thank you for your contribution. I've just checked and your commit doesn't appear to be signed-off. That's something we need before your Pull Request can be merged. Please see our contributing guide.
Tip: if you only have one commit so far then run: git commit --amend --signoff and then git push --force.

[alexellis]

Thanks for the PR, can you send one over to https://github.com/openfaas-incubator/ruby-http too?

Are there breaking changes involved in moving from Ruby 2.5 to 2.7? cc @kaihoffman @andyjeffries

[kaihoffman]

Hi @alexellis 2.7 implements all methods etc from previous versions, it simply adds new features. 3.0 next year will be the one with potential breaking issues.

[themoah]

@alexellis it's only security fixes and some major GC improvement (alongside JIT). All new features will be released with 3.0.

[alexellis]

Let's go for it then.