added composer installer signature check according to official composer docs

[DerDummePunkt]

You can never be too careful.
This PR replaces the portion of the Dockefiles RUN statement, which deals with installing composer, with a more secure version, which also checks the SHA384 checksum of the downloaded installer against a SHA384 checksum obtained from the official git repository of the composer project.

The intention is to provide one additional layer of secruity against man-in-the-middle / spoofing attacks.
It is possible that someone might capture or redirect https://getcomposer.org/installer during docker build and replace the composer install file with a malicous version, which, in turn, would install malicous composer packages.
I would consider it to be less likely that someone might capture both https://getcomposer.org/installer and https://composer.github.io/installer.sig at the same time, thus increasing security.

added composer installer signature check according to official composer docs : https://getcomposer.org/doc/faqs/how-to-install-composer-programmatically.md

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[dlen]

Hi @DerDummePunkt thanks for your contribution!! You are completely right and your PR looks just fine all tests are green :)