Add extra (unsupported) container build arch's (#4524)

There was a PR (#4370) to add i686/i386 support for Vaultwarden.
That specific PR was not a viable way of adding this.

This PR adds extra architectures for Debian based containers which we
will not support by default. Those images will not be build and pushed
to our container registries.

Added the following architectures:
 - linux/386
 - linux/ppc64le
 - linux/s390x

Again, there will be no major support for these architectures, but it
will allow people who use these architectures to build a Debian based
binary more easily

docker/Dockerfile.alpine

@@ -65,13 +65,14 @@ RUN mkdir -pv "${CARGO_HOME}" \
 RUN USER=root cargo new --bin /app
 WORKDIR /app
 
-# Shared variables across Debian and Alpine
+# Environment variables for Cargo on Alpine based builds
 RUN echo "export CARGO_TARGET=${RUST_MUSL_CROSS_TARGET}" >> /env-cargo && \
     # To be able to build the armv6 image with mimalloc we need to tell the linker to also look for libatomic
     if [[ "${TARGETARCH}${TARGETVARIANT}" == "armv6" ]] ; then echo "export RUSTFLAGS='-Clink-arg=-latomic'" >> /env-cargo ; fi && \
     # Output the current contents of the file
     cat /env-cargo
 
+# Configure the DB ARG as late as possible to not invalidate the cached layers above
 # Enable MiMalloc to improve performance on Alpine builds
 ARG DB=sqlite,mysql,postgresql,enable_mimalloc