Use COMPLEMENT_HOSTNAME_RUNNING_COMPLEMENT config for ExtraHosts in Docker (#825)

(doesn't fix anything specifically, just aligning to what we should be doing)

It looks like this regressed in #389 (previously, we we're using `HostnameRunningComplement` here)

Author: MadLittleMods

internal/docker/builder.go

@@ -32,13 +32,6 @@ import (
 	"github.com/matrix-org/complement/internal/instruction"
 )
 
-var (
-	// HostnameRunningDocker is the hostname of the docker daemon from the perspective of Complement.
-	HostnameRunningDocker = "localhost"
-	// HostnameRunningComplement is the hostname of Complement from the perspective of a Homeserver.
-	HostnameRunningComplement = "host.docker.internal"
-)
-
 const complementLabel = "complement_context"
 
 type Builder struct {

internal/docker/deployer.go

@@ -348,7 +348,7 @@ func deployImage(
 		// interact with a complement-controlled test server.
 		// Note: this feature of docker landed in Docker 20.10,
 		// see https://github.com/moby/moby/pull/40007
-		extraHosts = []string{"host.docker.internal:host-gateway"}
+		extraHosts = []string{fmt.Sprintf("%s:host-gateway", cfg.HostnameRunningComplement)}
 	}
 
 	for _, m := range cfg.HostMounts {