[browser.dockerfile] Fixed build issue

Oops - I had forgotten to build the Theia extensions, which caused a
problem at runtime.

Signed-off-by: Marc Dumais <marc.dumais@ericsson.com>

browser.Dockerfile

@@ -13,6 +13,7 @@ COPY . .
 # Download plugins and build application production mode
 # Use yarn autoclean to remove unnecessary files from package dependencies
 RUN yarn --pure-lockfile && \
+    yarn build:extensions && \
     yarn download:plugins && \
     yarn browser build && \
     yarn --production && \