Optimize release workflow.

- Split Debian and Alpine into different build matrix
  This starts building both Debian and Alpine based images at the same time
- Make use of Docker BuildKit, which improves speed also.
- Use BuildKit caching for Rust Cargo across docker images.
  This prevents downloading the same crates multiple times.
- Use Github Actions Services to start a docker registry, starting it
via the build script sometimes caused issues.
- Updated the Build workflow to use Ubuntu 20.04 which is more close to
the Bullseye Debian release regarding package versions.

docker/armv7/Dockerfile.alpine

@@ -1,3 +1,5 @@
+# syntax=docker/dockerfile:1
+
 # This file was generated using a Jinja2 template.
 # Please make your changes in `Dockerfile.j2` and then `make` the individual Dockerfiles.
 
@@ -32,12 +34,18 @@ FROM messense/rust-musl-cross:armv7-musleabihf as build
 ARG DB=sqlite,vendored_openssl
 
 # Build time options to avoid dpkg warnings and help with reproducible builds.
-ENV DEBIAN_FRONTEND=noninteractive LANG=C.UTF-8 TZ=UTC TERM=xterm-256color
+ENV DEBIAN_FRONTEND=noninteractive \
+    LANG=C.UTF-8 \
+    TZ=UTC \
+    TERM=xterm-256color \
+    CARGO_HOME="/root/.cargo" \
+    USER="root"
 
-# Don't download rust docs
-RUN rustup set profile minimal
 
-ENV USER "root"
+# Create CARGO_HOME folder and don't download rust docs
+RUN mkdir -pv "${CARGO_HOME}" \
+    && rustup set profile minimal
+
 ENV RUSTFLAGS='-C link-arg=-s'
 ENV CFLAGS_armv7_unknown_linux_musleabihf="-mfpu=vfpv3-d16"