fix: updated to golang 1.15.2 and consitant alpine 3.12

[sootysec]

Related issue

Proposed changes

Updated dockerfile-e2e to use golang 1.15.2 now that 1.13 is EOL. Updated other dockerfiles to use Alpine 3.12 to make all dockerfile consitant.

Checklist

• [x ] I have read the contributing guidelines.
• [ x] I have read the security policy.
• [x ] I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further comments

[aeneasr]

Awesome 🎉

Thank you for your contribution!