Create Debian package for beszel-agent (#497)

* Add Debian package to build

Signed-off-by: Daniel Lo Nigro <git@d.sb>

* Fix Lintian issues

Signed-off-by: Daniel Lo Nigro <git@d.sb>

* - Add prerm script to stop and disable service
- Add security/sandboxing options to systemd unit

Signed-off-by: Daniel Lo Nigro <git@d.sb>

---------

Signed-off-by: Daniel Lo Nigro <git@d.sb>

supplemental/debian/postinstall.sh

@@ -0,0 +1,49 @@
+#!/bin/sh
+set -e
+
+[ "$1" = "configure" ] || exit 0
+
+CONFIG_FILE=/etc/beszel-agent.conf
+SERVICE=beszel-agent
+SERVICE_USER=beszel
+
+. /usr/share/debconf/confmodule
+
+# This would normally be in the config control file, however this is currently
+# broken in goreleaser. Temporarily do it here.
+# https://github.com/goreleaser/goreleaser/issues/5487
+db_version 2.0
+db_input high beszel-agent/key || true
+db_go
+
+# Create group and user
+if ! getent group "$SERVICE_USER" >/dev/null; then
+	echo "Creating $SERVICE_USER group"
+	addgroup --quiet --system "$SERVICE_USER"
+fi
+
+if ! getent passwd "$SERVICE_USER" >/dev/null; then
+	echo "Creating $SERVICE_USER user"
+	adduser --quiet --system "$SERVICE_USER" \
+		--ingroup "$SERVICE_USER" \
+		--no-create-home \
+		--home /nonexistent \
+		--gecos "System user for $SERVICE"
+fi
+
+# Create config file if it doesn't already exist
+if [ ! -f "$CONFIG_FILE" ]; then
+	touch "$CONFIG_FILE"
+	chmod 0600 "$CONFIG_FILE"
+	chown "$SERVICE_USER":"$SERVICE_USER" "$CONFIG_FILE"
+fi;
+
+# Only add key to config if it's not already present
+if ! grep -q "^KEY=" "$CONFIG_FILE"; then
+	db_get beszel-agent/key
+	echo "KEY=$RET" > "$CONFIG_FILE"
+fi;
+
+deb-systemd-helper enable "$SERVICE".service
+systemctl daemon-reload
+deb-systemd-invoke start "$SERVICE".service || echo "could not start $SERVICE.service!"