Build fails when FROM image AS alias has capital letters

[BenHizak]

Actual behavior
Named build stages (FROM someimage as myname) fail when name begins with a Capital letter
Example:

1. FROM python:3.7.2 AS library ✔ builds successfully (note that there are no capital letters)
2. FROM python:3.7.2 AS Library ❌ fails (note that there is a capital letter)
   Error shown

Downloading base image Library               
error building image: could not parse reference
ERROR

Expected behavior
When using named stages, capital letters should not break the build process
To Reproduce
Please see this gist

[N4rm0]

@tejal29 PR #787 should fix this issue.
Edit: Just realized the issue lies in 3rd party code (moby/buildkit), so I am dropping my PR as the fix should be made in the vendor code.

[tejal29]

Awesome! we should update to latest moby/buildkit

[antechrestos]

@N4rm0 you should report your fix in vendor code because currently the problem is still present

Upgrading to the latest moby/buildkit will not fix the issue

[antechrestos]

@tejal29 I implemented a small fix by implementing it in the resolution in order not to change third party library behaviour