feat(installer): build worker source over SSH

internal/installer/harness/harness_test.go

@@ -235,6 +235,21 @@ func TestSetDockerBin(t *testing.T) {
 	SetDockerBin("")
 }
 
+func TestDockerDNSOverride(t *testing.T) {
+	t.Setenv("RSMON_TEST_DOCKER_DNS", "")
+	if got := dockerDNS(); len(got) != 0 {
+		t.Fatalf("dockerDNS() with empty env = %v, want none", got)
+	}
+	t.Setenv("RSMON_TEST_DOCKER_DNS", "8.8.8.8, 1.1.1.1")
+	if got := dockerDNS(); len(got) != 4 || got[0] != "--dns" || got[1] != "8.8.8.8" || got[3] != "1.1.1.1" {
+		t.Fatalf("dockerDNS() = %v", got)
+	}
+	t.Setenv("RSMON_TEST_DOCKER_DNS", "  ,,")
+	if got := dockerDNS(); len(got) != 0 {
+		t.Fatalf("dockerDNS() with blank entries = %v", got)
+	}
+}
+
 // writeStubDocker installs a fake docker binary that records its argv to
 // logPath and returns the recorded path. The stub succeeds for build,
 // network, run, and teardown calls; `port` fails so Start fails after