INTL (icu) php extension : cannot find config.m4

I am trying to add INTL extension by following (How do I compile additional PHP extensions?).

I copied the icu folder (icu v64.2) in app.
Then, in ssh terminal :

root@fdda516f2f9b:/# apt-get install -y libicu-dev
Reading package lists… Done
Building dependency tree
Reading state information… Done
libicu-dev is already the newest version.

root@fdda516f2f9b:/app/icu/source# /opt/php/7.3.2/bin/phpize

Cannot find config.m4.

Make sure that you run ‘/opt/php/7.3.2/bin/phpize’ in the top level source directory of the module