Simplify startup and dependencies (#165)

Currently there is a whole dependency forest just to start a couple of
startup scripts in order.
Additionally all the services also have a dependency on all the startup
scripts.
This is unnecessary complex, simplify it:

Move startup scripts to /etc/s6-overlay/startup.d
Create oneshot service named "startup" which executes all files in
/etc/s6-overlay/startup.d in alphabetical order
Remove various oneshot scripts from longrun service dependencies
Add "startup" oneshot service to longrun service dependencies

The oneshot service to start all scripts in /etc/s6-overlay/startup.d
could be included in the baseimage after some testing and if that's
something sdr-enthusiasts is interested in.
The script was tested against an empty or non-existent directory as well
and just exits 0 in that case.
If one of started scripts exits with an error, it will announce it and
propagate the error.
This preserves the behaviour of the container startup being aborted in
that case.

Dockerfile

@@ -32,7 +32,10 @@ ENV S6_BEHAVIOUR_IF_STAGE2_FAILS=2 \
 
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
 
-COPY rootfs/ /
+# only copy files necessary for the build, copy whole rootfs later
+# this improves build caching when changing service and startup scripting
+COPY rootfs/tar1090-install.sh /
+COPY rootfs/etc/nginx.tar1090 /etc/nginx.tar1090
 
 # add telegraf binary
 ##telegraf##COPY --from=telegraf /usr/bin/telegraf /usr/bin/telegraf
@@ -178,6 +181,8 @@ RUN set -x && \
     popd && \
     rm -rf /tmp/*
 
+COPY rootfs/ /
+
 EXPOSE 80/tcp
 
 # Add healthcheck

rootfs/etc/s6-overlay/s6-rc.d/startup/up

@@ -0,0 +1,2 @@
+#!/bin/sh
+exec /etc/s6-overlay/scripts/startup

rootfs/etc/s6-overlay/scripts/readsb

@@ -323,4 +323,4 @@ if chk_enabled "$PROMETHEUS_ENABLE"; then
 fi
 
 # shellcheck disable=SC2086
-"${s6wrap[@]}" "${READSB_BIN}" "${READSB_CMD[@]}" $READSB_EXTRA_ARGS
+exec "${s6wrap[@]}" "${READSB_BIN}" "${READSB_CMD[@]}" $READSB_EXTRA_ARGS

rootfs/etc/s6-overlay/scripts/startup

@@ -0,0 +1,18 @@
+#!/command/with-contenv bash
+# shellcheck shell=bash disable=SC1091,SC2076
+
+source /scripts/common
+
+SDIR=/etc/s6-overlay/startup.d
+
+# exit 0 for nonexistent or empty directory
+if ! [[ -d "$SDIR" ]] || [[ -z "$(ls "$SDIR")" ]]; then
+    exit 0
+fi
+
+for NAME in "$SDIR"/*; do
+    if ! s6wrap --quiet --prepend="$NAME" --timestamps --args "$NAME"; then
+        s6wrap --quiet --prepend=startup --timestamps --args echo Error running "$NAME"
+        exit 1
+    fi
+done