Bind 0.0.0.0 to IPv4 only (closes #528)

Go listens on any unspecified address with one socket that answers
IPv4 and IPv6, so BIND_ADDRESS=0.0.0.0 also bound every IPv6 address,
which the README says it does not. The listener now uses tcp4 for an
IPv4 address, written plain or IPv4-mapped; :: still binds both. The
server's start hook suppresses contextcheck, as the other start hooks
do.

The image's health check asked for localhost, which wget reached over
::1 only. It now asks for the container's own address: its IPv4
address, or its IPv6 address on an IPv6-only network. A loopback bind
answers neither. The README says such a network needs ::, drops the
bindv6only condition from :: (Go sets that socket option itself) and
names an ss check.

Model: opus-5-5

Dockerfile

@@ -274,8 +274,17 @@ EXPOSE 8080
 # port.
 ENV BIND_ADDRESS=0.0.0.0
 
+# The probe requests the container's own address, not loopback, so a
+# listener that answers only on loopback (an empty or loopback
+# BIND_ADDRESS) fails it. On a network with IPv4, hostname -i prints the
+# container's IPv4 address, which 0.0.0.0 and :: both answer. On an
+# IPv6-only network it prints none, and the probe requests the IPv6
+# address /etc/hosts lists under the container's hostname instead, which
+# only :: answers.
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
-    CMD wget --no-verbose --tries=1 --spider http://localhost:8080/.well-known/healthcheck || exit 1
+    CMD addr="$(hostname -i 2>/dev/null | cut -d ' ' -f 1)"; \
+        [ -n "$addr" ] || addr="[$(getent hosts "$(hostname)" | cut -d ' ' -f 1)]"; \
+        wget --no-verbose --tries=1 --spider "http://$addr:8080/.well-known/healthcheck" || exit 1
 
 ENTRYPOINT ["/usr/local/bin/docker-entrypoint.sh"]
 CMD ["/app/webhooker"]