Cannot create cache directory

Shopware 6 (German)
1

Hey there!

Ich habe beim De-/Installieren von Plugins massive Probleme. Beim Aufruf von bin/console plugin:refresh erhielt ich dann folgende Fehlermeldungen:

The '/var/www/html/custom/plugins/PickwareDhl/composer.json' has some warnings:
The version field is present, it is recommended to leave it out if the package is published on Packagist.
<warning>Cannot create cache directory /usr/local/bin/composer/cache/repo/https---repo.packagist.org/, or directory is not writable. Proceeding without cache</warning>
<warning>Cannot create cache directory /usr/local/bin/composer/cache/files/, or directory is not writable. Proceeding without cache</warning>
<warning>Could not scan for classes inside "/var/www/html/custom/plugins/PickwareDhl/vendor/jdorn/sql-formatter/lib" which does not appear to be a file nor a folder</warning>
<warning>Could not scan for classes inside "/var/www/html/custom/plugins/PickwareDhl/vendor/dompdf/dompdf/lib/" which does not appear to be a file nor a folder</warning>
<warning>Could not scan for classes inside "/var/www/html/custom/plugins/PickwareDhl/vendor/symfony/intl/Resources/stubs" which does not appear to be a file nor a folder</warning>
<warning>Could not scan for classes inside "/var/www/html/custom/plugins/PickwareDhl/vendor/symfony/polyfill-intl-normalizer/Resources/stubs" which does not appear to be a file nor a folder</warning>
<warning>Could not scan for classes inside "/var/www/html/custom/plugins/PickwareDhl/vendor/symfony/polyfill-php73/Resources/stubs" which does not appear to be a file nor a folder</warning>
<warning>Could not scan for classes inside "/var/www/html/custom/plugins/PickwareDhl/vendor/symfony/polyfill-php80/Resources/stubs" which does not appear to be a file nor a folder</warning>
Attention!
The '/var/www/html/custom/plugins/StripeShopwarePayment/composer.json' has some warnings:
The version field is present, it is recommended to leave it out if the package is published on Packagist.
<warning>Cannot create cache directory /usr/local/bin/composer/cache/repo/https---repo.packagist.org/, or directory is not writable. Proceeding without cache</warning>
<warning>Cannot create cache directory /usr/local/bin/composer/cache/files/, or directory is not writable. Proceeding without cache</warning>
Attention!
The '/var/www/html/custom/plugins/SwagExtensionStore/composer.json' has some warnings:
The version field is present, it is recommended to leave it out if the package is published on Packagist.
<warning>Cannot create cache directory /usr/local/bin/composer/cache/repo/https---repo.packagist.org/, or directory is not writable. Proceeding without cache</warning>
<warning>Cannot create cache directory /usr/local/bin/composer/cache/files/, or directory is not writable. Proceeding without cache</warning>

Ich verwende das aktuelle Image von Dockware dockware/essentials:latest. Logischerweise kann unter /usr/local/bin/ kein Cache-Directory sein. Deshalb habe ich versucht den Pfad sowohl über die composer.json im Shopware Root als auch unter ~/.composer/config.json anzupassen:

{
    "config": {
        "cache-dir": "/var/www/.composer/cache"
    }
}

Leider ohne Erfolg. Wie kann ich Shopware bzw. Composer sagen welches Cache-Dir zu verwenden ist?