Composer Install (own Container) with Docker missing PHP Extensions
You may use platform settings to mimic your PHP container configuration. This will work similar to --ignore-platform-reqs switch (it will use PHP and extensions configured in composer.json instead of real info from current PHP installation), but it gives you more granular control. Instead of "ignore all platform requirements checks" you may say "I really have bcmath installed, trust me". All other requirements will be checked, so you still be warned if new requirement will pop-up.
"config": { "platform": { "php": "7.1", "ext-bcmath": "*" }},
You need PHP + PHP Extensions + Composer in the same(!) container = DevContainer.
Simply install Composer using the command provided here.