Make sure we dont have hidden options for backend and pipeline compiler (#2123)

move options based on **os.Getenv** into flags

---------
*Sponsored by Kithara Software GmbH*

pipeline/backend/docker/flags.go

@@ -0,0 +1,59 @@
+// Copyright 2023 Woodpecker Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package docker
+
+import (
+	"github.com/urfave/cli/v2"
+)
+
+var Flags = []cli.Flag{
+	&cli.StringFlag{
+		EnvVars: []string{"WOODPECKER_BACKEND_DOCKER_HOST", "DOCKER_HOST"},
+		Name:    "backend-docker-host",
+		Usage:   "path to docker socket or url to the docker server",
+	},
+	&cli.StringFlag{
+		EnvVars: []string{"WOODPECKER_BACKEND_DOCKER_API_VERSION", "DOCKER_API_VERSION"},
+		Name:    "backend-docker-api-version",
+		Usage:   "the version of the API to reach, leave empty for latest.",
+	},
+	&cli.StringFlag{
+		EnvVars: []string{"WOODPECKER_BACKEND_DOCKER_CERT_PATH", "DOCKER_CERT_PATH"},
+		Name:    "backend-docker-cert",
+		Usage:   "path to load the TLS certificates for connecting to docker server",
+	},
+	&cli.BoolFlag{
+		EnvVars: []string{"WOODPECKER_BACKEND_DOCKER_TLS_VERIFY", "DOCKER_TLS_VERIFY"},
+		Name:    "backend-docker-tls-verify",
+		Usage:   "enable or disable TLS verification for connecting to docker server",
+		Value:   true,
+	},
+	&cli.BoolFlag{
+		EnvVars: []string{"WOODPECKER_BACKEND_DOCKER_ENABLE_IPV6"},
+		Name:    "backend-docker-ipv6",
+		Usage:   "backend docker enable IPV6",
+		Value:   false,
+	},
+	&cli.StringFlag{
+		EnvVars: []string{"WOODPECKER_BACKEND_DOCKER_NETWORK"},
+		Name:    "backend-docker-network",
+		Usage:   "backend docker network",
+	},
+	&cli.StringFlag{
+		EnvVars: []string{"WOODPECKER_BACKEND_DOCKER_VOLUMES"},
+		Name:    "backend-docker-volumes",
+		Usage:   "backend docker volumes (comma separated)",
+	},
+}