Fix dockerfile warnings in browser.Dockerfile

[teksturi]

What it does

When building this dockerfile I get warnings with docker

WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 2)
WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 29)

How to test

docker build -t theia -f browser.Dockerfile .

Review checklist

• [x] as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines