Add bcmath php extension

[davidglezz]

Questions	Answers
Description?	The bcmath extension is used and unit tests fail if it is not installed, why not include it?
Type?	improvement
BC breaks?	no
Deprecations?	no
Fixed ticket?	no
How to test?	Image builds and runs Prestashop as expected
Possible impacts?	I think no

When I do:

docker compose up -d
docker compose exec prestashop-git ./vendor/phpunit/phpunit/phpunit -c tests/Unit/phpunit.xml

I get:

1) Tests\Unit\Adapter\Tax\TaxComputerTest::testComputePriceWithoutTaxes with data set #5 (16.43, '8.333333333', '7.157376000000', 12)
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'7.157376000000'
+'7.157376391000'

with this patch I think it would no longer be a issue.