Upgrade dockerfile version

[dvmarkusvogl]

Observed behavior

• Our dockerfile-version on merging is 2.0 (no matter what your version in your file is)
• The dockerfile 3.0 spec was released in 2017-01-18, so 5 years ago. Yet we don't support version 3 syntax, which would offer things like templating to save duplicated code, or easier networking as well as some other features

Expected behavior

• Give warnings if people use 2.* only commands like extends
• Set version to 3. if it's compatible.
• The generator not crashing if I use the extension fields templating syntax from above, because it's confused by & and <<: *

[gayatrisingh31]

Hi @dvmarkusvogl can i do this?

no you are not allowed to do it now