[proxy] More flexible container name -> hostname derivation

Provides to new two new flags: `--hostname-match <regexp>`
and `--hostname-replacement <replacement>`. When launching a container, its name
matched against regular expression `<regexp>`. Then, based on that match,
`<replacement>` will be used to generate a hostname, which will ultimately be
handed over to weaveDNS for registration.

The substitution is implemented with
https://golang.org/pkg/regexp/#Regexp.ReplaceAllString . Thus, references to
regexp substitution groups should be prepended with a dollar sign.

For instance, if we provide `--hostname-match '^aws-[0-9]+-(.*)$'` and
`--hostname-replacement 'my-app-$1'`, running a container named
`aws-12798186823-foo` through WeaveProxy will lead to WeaveDNS registering
`my-app-foo`.

Closes #1018

proxy/create_container_interceptor.go

@@ -58,7 +58,9 @@ func (i *createContainerInterceptor) InterceptRequest(r *http.Request) error {
 		if err := i.setWeaveWaitEntrypoint(container.Config); err != nil {
 			return err
 		}
-		if err := i.setWeaveDNS(&container, r.URL.Query().Get("name")); err != nil {
+		containerName := r.URL.Query().Get("name")
+		hostname := i.proxy.hostnameMatchRegexp.ReplaceAllString(containerName, i.proxy.HostnameReplacement)
+		if err := i.setWeaveDNS(&container, hostname); err != nil {
 			return err
 		}
 	}

proxy/proxy.go

@@ -15,11 +15,13 @@ import (
 )
 
 const (
-	defaultCaFile   = "ca.pem"
-	defaultKeyFile  = "key.pem"
-	defaultCertFile = "cert.pem"
-	dockerSock      = "/var/run/docker.sock"
-	dockerSockUnix  = "unix://" + dockerSock
+	defaultCaFile               = "ca.pem"
+	defaultKeyFile              = "key.pem"
+	defaultCertFile             = "cert.pem"
+	dockerSock                  = "/var/run/docker.sock"
+	dockerSockUnix              = "unix://" + dockerSock
+	substitutionPatternName     = "pattern"
+	substitutionReplacementName = "replacement"
 )
 
 var (
@@ -29,19 +31,22 @@ var (
 )
 
 type Config struct {
-	ListenAddrs    []string
-	NoDefaultIPAM  bool
-	NoRewriteHosts bool
-	TLSConfig      TLSConfig
-	Version        string
-	WithDNS        bool
-	WithoutDNS     bool
+	HostnameMatch       string
+	HostnameReplacement string
+	ListenAddrs         []string
+	NoDefaultIPAM       bool
+	NoRewriteHosts      bool
+	TLSConfig           TLSConfig
+	Version             string
+	WithDNS             bool
+	WithoutDNS          bool
 }
 
 type Proxy struct {
 	Config
-	client         *docker.Client
-	dockerBridgeIP string
+	client              *docker.Client
+	dockerBridgeIP      string
+	hostnameMatchRegexp *regexp.Regexp
 }
 
 func NewProxy(c Config) (*Proxy, error) {
@@ -65,6 +70,12 @@ func NewProxy(c Config) (*Proxy, error) {
 		p.dockerBridgeIP = string(dockerBridgeIP)
 	}
 
+	p.hostnameMatchRegexp, err = regexp.Compile(c.HostnameMatch)
+	if err != nil {
+		err := fmt.Errorf("Incorrect hostname match '%s': %s", c.HostnameMatch, err.Error())
+		return nil, err
+	}
+
 	return p, nil
 }