Fix TODO in dockerfile (#528)

* Fix TODO in dockerfile and also add script to build extensions

* try to build pg_duckdb

* fix functions

* Check number of processors

* Check number of processors again

* Check number of processors again

* build arg for extension

* add submodule

* check compile time

* fix actions

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Co-authored-by: Admire Nyakudya <mazano@za-nyakudya-mac.local>