ci: enhance build process (#2755)

This new script is a clean way of handling the installation of packages.
I think the huge `RUN` command in `Dockerfile` was hard to read and
maintain.

Using a script is a non-issue, as the image is rebuilt whenever the
script is touched.

Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

target/scripts/build/packages.sh

@@ -0,0 +1,112 @@
+#! /bin/bash
+
+# -eE :: exit on error (do this in functions as well)
+# -u  :: show (and exit) when using unset variables
+# -o pipefail :: exit on error in pipes
+set -eE -u -o pipefail
+
+# shellcheck source=../helpers/log.sh
+source /usr/local/bin/helpers/log.sh
+
+_log_level_is 'trace' && QUIET='-y' || QUIET='-qq'
+
+function _pre_installation_steps
+{
+  _log 'info' 'Starting package installation'
+  _log 'debug' 'Running pre-installation steps'
+
+  _log 'trace' 'Updating package signatures'
+  apt-get "${QUIET}" update
+
+  _log 'trace' 'Installing packages that are needed early'
+  apt-get "${QUIET}" install --no-install-recommends apt-utils 2>/dev/null
+
+  _log 'trace' 'Upgrading packages'
+  apt-get "${QUIET}" upgrade
+}
+
+function _install_postfix
+{
+  _log 'debug' 'Installing Postfix'
+
+  _log 'warn' 'Applying workaround for Postfix bug (see https://github.com//issues/2023#issuecomment-855326403)'
+
+  # Debians postfix package has a post-install script that expects a valid FQDN hostname to work:
+  mv /bin/hostname /bin/hostname.bak
+  echo "echo 'docker-mailserver.invalid'" >/bin/hostname
+  chmod +x /bin/hostname
+  apt-get "${QUIET}" install --no-install-recommends postfix
+  mv /bin/hostname.bak /bin/hostname
+}
+
+function _install_packages
+{
+  _log 'debug' 'Installing all packages now'
+
+  declare -a DOVECOT_PACKAGES ANTI_VIRUS_SPAM_PACKAGES
+  declare -a CODECS_PACKAGES MISCELLANEOUS_PACKAGES
+  declare -a POSTFIX_PACKAGES MAIL_PROGRAMS_PACKAGES
+
+  DOVECOT_PACKAGES=(
+    dovecot-core dovecot-fts-xapian dovecot-imapd
+    dovecot-ldap dovecot-lmtpd dovecot-managesieved
+    dovecot-pop3d dovecot-sieve dovecot-solr
+  )
+
+  ANTI_VIRUS_SPAM_PACKAGES=(
+    amavisd-new clamav clamav-daemon
+    fail2ban pyzor razor spamassassin
+  )
+
+  CODECS_PACKAGES=(
+    altermime arj bzip2
+    cabextract cpio file
+    gzip lhasa liblz4-tool
+    lrzip lzop nomarch
+    p7zip-full pax rpm2cpio
+    unrar-free unzip xz-utils
+  )
+
+  MISCELLANEOUS_PACKAGES=(
+    apt-transport-https binutils bsd-mailx
+    ca-certificates curl dbconfig-no-thanks
+    dumb-init ed gamin gnupg iproute2
+    libdate-manip-perl libldap-common
+    libmail-spf-perl libnet-dns-perl
+    locales logwatch netcat-openbsd
+    nftables rsyslog supervisor
+    uuid whois
+  )
+
+  POSTFIX_PACKAGES=(
+    pflogsumm postgrey postfix-ldap
+    postfix-pcre postfix-policyd-spf-python postsrsd
+  )
+
+  MAIL_PROGRAMS_PACKAGES=(
+    fetchmail opendkim opendkim-tools
+    opendmarc libsasl2-modules sasl2-bin
+  )
+
+  apt-get "${QUIET}" --no-install-recommends install \
+    "${DOVECOT_PACKAGES[@]}" \
+    "${ANTI_VIRUS_SPAM_PACKAGES[@]}" \
+    "${CODECS_PACKAGES[@]}" \
+    "${MISCELLANEOUS_PACKAGES[@]}" \
+    "${POSTFIX_PACKAGES[@]}" \
+    "${MAIL_PROGRAMS_PACKAGES[@]}"
+}
+
+function _post_installation_steps
+{
+  _log 'debug' 'Running post-installation steps (cleanup)'
+  apt-get "${QUIET}" clean
+  rm -rf /var/lib/apt/lists/*
+
+  _log 'info' 'Finished installing packages'
+}
+
+_pre_installation_steps
+_install_postfix
+_install_packages
+_post_installation_steps