avoid clock problems on some Docker setups

In particular, this was happening out of the box on Raspberry Pi OS Lite
20210304, as reported by ironoxidizer@gmail.com here:
https://groups.google.com/g/moonfire-nvr-users/c/2j9LvfFl2u8/m/tJcNS2WfCQAJ

*   adjust main.rs to make the problem more obvious
*   mention it in the troubleshooting guide
*   sidestep it in the nvr docker wrapper script

also just use --networking=host rather than --publish (avoiding a proxy
process). I'm using Docker to simplify the build and deployment process,
not as a security boundary, so just do the simpler thing.

server/src/main.rs

@@ -137,6 +137,15 @@ fn panic_hook(p: &std::panic::PanicInfo) {
 }
 
 fn main() {
+    if let Err(e) = nix::time::clock_gettime(nix::time::ClockId::CLOCK_MONOTONIC) {
+        eprintln!(
+            "clock_gettime failed: {}\n\n\
+                   This indicates a broken environment. See the troubleshooting guide.",
+            e
+        );
+        std::process::exit(1);
+    }
+
     let args = Args::from_args();
     let mut h = mylog::Builder::new()
         .set_format(