Add optional GPS reference clock via gpsd

[tillo]

Adds optional GPS reference-clock support, mirroring the existing PTP0 opt-in pattern.

Motivation

Several of us run a stratum-1 NTP-Pool / OpenNIC server backed by a serial GPS receiver (USB UART GPS dongle, hat-mounted GPS on a router/SBC, etc.). The image already has all the chrony machinery for a serial refclock, so the only missing piece is bringing up gpsd against the device and adding a refclock SHM ... line to chrony.conf. That's what this PR does — about ten lines in the entrypoint.

I've been maintaining it as an out-of-tree fork for a year; upstreaming so anyone else with a serial GPS can run a stratum-1 from this image without a custom build.

Behaviour

Opt-in via a single env var, same shape as ENABLE_NTS/ENABLE_SYSCLK/the PTP0 device-passthrough:

devices:
  - /dev/ttyUSB0:/dev/ttyUSB0
environment:
  - GPS_DEVICE=/dev/ttyUSB0
  - GPS_BAUD=9600              # default 9600
  - GPS_REFCLOCK_OFFSET=0.0    # default 0.0; serial-latency compensation
  - GPS_PPS=false              # set true to also consume PPS via SHM seg 1

When GPS_DEVICE is set and the device exists, the entrypoint launches
gpsd -N -F /run/chrony/gpsd.sock -s ${GPS_BAUD} ${GPS_DEVICE} in background and appends refclock SHM 0 refid GPS offset … precision 1e-1 to chrony.conf. With GPS_PPS=true a second refclock SHM 1 refid PPS precision 1e-7 prefer line is added — useful for receivers whose PPS pin is wired through and gpsd is configured to publish PPS samples on SHM segment 1.

When unset, gpsd is installed but never started.

Image-size impact for non-GPS users

gpsd is added to the apk add line so the image grows by a few MB regardless of whether GPS is enabled. I considered making it a separate build target / Dockerfile.gps so the default image stays unchanged, but the unified pattern (ENABLE_NTS, ENABLE_SYSCLK, PTP0 device-passthrough — all single-image, runtime-opt-in) seemed more in line with how the rest of the image works. Happy to split into a gpsd-tagged variant instead if you prefer.

Permissions note

The serial device must be readable by the chrony user. Documented in the README — easiest is chmod g+r /dev/ttyUSB0 + chown :dialout /dev/ttyUSB0 on the host plus group_add: [dialout] on the container; or run the container as root.

Tested

In production on a Mediatek MT7988 (BPI-R4) + USB-UART GPS, feeding an NTP-Pool stratum-1 server. The fork has been running this code path since April; the recent improvement in this PR (the [ -e "${GPS_DEVICE}" ] guard) is to make a missing/unplugged GPS not abort chronyd so the container still serves stratum-2 from NTP_SERVERS until the hardware is replaced.