upaas: health check, settings checked at start, README section (closes #59)

The image's HEALTHCHECK requests /.well-known/healthcheck through
nginx on the port from PORT, so it fails unless both processes answer.
The backend reads PORT and DEBUG with strconv instead of viper, which
turned a bad PORT into 0 and a bad DEBUG into false. Those, and a
BIND_ADDRESS that is not an IP address, now stop the start with an
error naming the variable; the TRUSTED_PROXIES error names it too.
bin/entrypoint.sh also refuses a container PORT outside 1 to 65535,
or 8081, where the backend listens, naming PORT. README.md gains
"Running under upaas". Its first-run steps create the host directory
owned by uid 1000, so the image changes no ownership.

Model: opus-5-5

bin/entrypoint.sh

@@ -10,8 +10,9 @@ set -u
 
 # PORT is the public port nginx listens on, 8080 when unset or empty.
 # nginx would take a value such as localhost or unix:/tmp/x.sock as an
-# address and start anyway, so anything but digits stops the container
-# here, before either process starts.
+# address and start anyway, and reports a bad port without naming
+# PORT, so a value that is not a usable port stops the container here,
+# before either process starts.
 export PORT="${PORT:-8080}"
 case "$PORT" in
     *[!0-9]*)
@@ -19,6 +20,17 @@ case "$PORT" in
         exit 1
         ;;
 esac
+# The length is checked first because, for a number too big for it,
+# the shell's test prints an error and is false, so the range checks
+# alone would let it through.
+if [ "${#PORT}" -gt 5 ] || [ "$PORT" -lt 1 ] || [ "$PORT" -gt 65535 ]; then
+    echo "entrypoint: PORT must be from 1 to 65535, not '$PORT'" >&2
+    exit 1
+fi
+if [ "$PORT" -eq 8081 ]; then
+    echo "entrypoint: PORT cannot be 8081, netwatch-server listens there" >&2
+    exit 1
+fi
 
 # A stop signal is only noted here; the loop below acts on it.
 stop_requested=""