Re-Create /usr/bin/php

As of PHP8 under Alpine `/usr/bin/php`  does not exist anymore in order to allow co-existence with `php7` packages.
The composer therefore fails when trying to execute `php`.

Weirdly this issue first surfaced with PR mriedmann#285 but not earlier - not sure why.

See https://gitlab.alpinelinux.org/alpine/aports/-/issues/12308

Dockerfile

@@ -69,6 +69,7 @@ ARG HUMHUB_VERSION
 ARG BUILD_DEPS
 
 RUN apk add --no-cache --update $BUILD_DEPS && \
+    ln -s /usr/bin/php81 /usr/bin/php && \
     rm -rf /var/cache/apk/*
 
 COPY --from=builder-composer /usr/bin/composer /usr/bin/composer