WIP: Speed up build

[oscarkraemer]

Proposed changes

• This changes tires to speed up the build process.
• It assumes that requirements.txt will seldom be changed and therefor it can use a cached version.
• Removing the git clone, as far as I can see there is no use of this clone BUT I might be wrong.
• There should be no changes to https://csc-guide-preview.rahtiapp.fi/origin/testing-speedup/
• It assumes that the /docs will be changed most frequently and therefor everything that is dependent on it is removed to the end of the Dockerfile

Checklist before requesting a review

• [x] I have followed the instructions in the Contributing and Styleguide documents.
• [x] My pull request passes all tests.
• [x] I have included a link to the appropriate preview page (select your branch from the list).