feat(ops/builderball): introduce bouncing Nix cache proxy

Adds a Nix cache proxy which can be used to send a Nix cache lookup to the first
available cache that has the given NAR. We will use this for dynamically created
builders.

Relates to b/432.

Change-Id: If970d2393e43ba032b5b7d653f2b92f6ac0eab63
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12949
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: sterni <sternenseemann@systemli.org>

ops/builderball/config/config.go

@@ -0,0 +1,42 @@
+package config
+
+import (
+	"flag"
+	"strings"
+)
+
+var (
+	Host     = flag.String("host", "localhost", "host on which to listen for incoming requests")
+	Port     = flag.Int("port", 2243, "port on which to listen for incoming requests")
+	Debug    = flag.Bool("debug", false, "whether debug logging should be enabled")
+	JSON     = flag.Bool("json", false, "whether logging should be in JSON format")
+	Ticker   = flag.Int("ticker", 5, "interval in seconds between statistics tickers")
+	Priority = flag.Int("priority", 50, "Nix cache priority with which to serve clients")
+
+	Tailscale = flag.Bool("tailscale", false, "whether caches should be discovered on Tailscale")
+	TSTag     = flag.String("tailscale-tag", "tag:nix-cache", "Tailscale tag to use for discovery")
+
+	CachePort = flag.Int("cache-port", 80, "port at which to connect to binary cache on each node")
+	CacheHost = flag.String("cache-host", "", "Host header to send to each binary cache")
+
+	Caches []string
+)
+
+type stringSliceFlag []string
+
+func (s *stringSliceFlag) String() string {
+	if len(*s) == 0 {
+		return "[ ]"
+	}
+
+	return "[ " + strings.Join(*s, " ") + " ]"
+}
+
+func (s *stringSliceFlag) Set(value string) error {
+	*s = append(*s, value)
+	return nil
+}
+
+func init() {
+	flag.Var((*stringSliceFlag)(&Caches), "cache", "Upstream cache URL (can be specified multiple times)")
+}