Bumped alpine version in the Dockerfile (#155)
Bumped alpine version in the Dockerfile (#155)

With the original version I was getting the following error:
```ERROR:  Could not find a valid gem 'mdl' (>= 0), here is why:
          Unable to download data from https://rubygems.org/ - SSL_connect returned=1 errno=0 state=error: certificate verify failed (https://api.rubygems.org/specs.4.8.gz)```
Bumping Alpine to latest version fixed it.
Also removed an unnecessary apk update.
DavidAnson/markdownlint-ruby,markdownlint/markdownlint,mivok/markdownlint