Upd: Update documentation - fix typos, make it more explicit we use D…

…ebian as default

Dockerfile

@@ -1,7 +1,14 @@
 # syntax=docker/dockerfile:1.6
 
+# Note:
+# The BASE_IMAGE can be changed for this docker image. In fact, it will be. Check .github/workflows/master.yml.
+# This image is automatically built with Debian, Ubuntu and Alpine as underlying systems. Each of these has its
+# own advantages and shortcomings. In essence:
+#
+# - use Alpine if you're strapped for space. But beware it uses MUSL LIBC, so unicode support might be an issue.
+# - use Debian if you're interested in the greatest cross-platform compatibility. It is larger than Alpine, though.
+# - use Ubuntu if, well, Ubuntu is your thing and you're used to Ubuntu ecosystem.
 ARG BASE_IMAGE=debian:bookworm-slim
-# ARG BASE_IMAGE=ubuntu:jammy
 
 FROM ${BASE_IMAGE} AS build-scripts
 COPY ./build-scripts ./build-scripts
@@ -13,8 +20,6 @@ ARG TARGETPLATFORM
 # Install supervisor, postfix
 # Install postfix first to get the first account (101)
 # Install opendkim second to get the second account (102)
-# --mount=type=cache,target=/var/cache/apk,sharing=locked,id=var-cache-apk-$TARGETPLATFORM \
-# --mount=type=cache,target=/etc/apk/cache,sharing=locked,id=etc-apk-cache-$TARGETPLATFORM \
 RUN --mount=type=cache,target=/var/cache/apt,sharing=locked,id=var-cache-apt-$TARGETPLATFORM \
  --mount=type=cache,target=/var/lib/apt,sharing=locked,id=var-lib-apt-$TARGETPLATFORM \
  --mount=type=tmpfs,target=/var/cache/apk \