Make listen port configurable (#230)

Co-authored-by: Tobias Hess <tobias.hess@energiekoppler.com>

internal/config_test.go

@@ -37,6 +37,7 @@ func TestConfigDefaults(t *testing.T) {
 	assert.False(c.MatchWhitelistOrDomain)
 	assert.Equal("/_oauth", c.Path)
 	assert.Len(c.Whitelist, 0)
+	assert.Equal(c.Port, 4181)
 
 	assert.Equal("select_account", c.Providers.Google.Prompt)
 }
@@ -51,6 +52,7 @@ func TestConfigParseArgs(t *testing.T) {
 		"--rule.1.rule=PathPrefix(`/one`)",
 		"--rule.two.action=auth",
 		"--rule.two.rule=\"Host(`two.com`) && Path(`/two`)\"",
+		"--port=8000",
 	})
 	require.Nil(t, err)
 
@@ -58,6 +60,7 @@ func TestConfigParseArgs(t *testing.T) {
 	assert.Equal("cookiename", c.CookieName)
 	assert.Equal("csrfcookiename", c.CSRFCookieName)
 	assert.Equal("oidc", c.DefaultProvider)
+	assert.Equal(8000, c.Port)
 
 	// Check rules
 	assert.Equal(map[string]*Rule{