watch: only allow a single instance per-project

This is a good place to start introducing (local) exclusivity
to Compose. Now, when `alpha watch` launches, it will check for
the existence of a PID file in the user XDG runtime directory,
and create one if the existing one is stale or does not exist.
If the PID file exists and is valid, an error is returned and
Compose exits.

A slight tweak to the experimental remote Git loader has been
made to use the XDG package for consistency.

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

internal/locker/pidfile.go

@@ -0,0 +1,41 @@
+/*
+   Copyright 2023 Docker Compose CLI 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 locker
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/adrg/xdg"
+	"github.com/docker/docker/pkg/pidfile"
+)
+
+type Pidfile struct {
+	path string
+}
+
+func NewPidfile(projectName string) (*Pidfile, error) {
+	path, err := xdg.RuntimeFile(fmt.Sprintf("docker-compose.%s.pid", projectName))
+	if err != nil {
+		return nil, err
+	}
+	return &Pidfile{path: path}, nil
+}
+
+func (f *Pidfile) Lock() error {
+	return pidfile.Write(f.path, os.Getpid())
+}