Add docker-compose.yml for deploying upaas (closes #223)

The compose file builds the image from this repo, mounts the Docker
socket and HOST_DATA_DIR (passed to upaas as UPAAS_HOST_DATA_DIR),
reads settings from .env, and restarts unless stopped. The port is
published on 127.0.0.1 only, for a TLS-terminating proxy in front, and
PORT is pinned to 8080 so .env cannot move upaas off the port mapping
and the healthcheck, which uses the runtime image's busybox wget.

upaas now refuses to start when UPAAS_HOST_DATA_DIR is set to a
relative path; when unset it still falls back to the data directory.

The README's plain-HTTP Compose example becomes a short deploy section
that points at the file. .env is added to .dockerignore.

Model: opus-5-5

internal/config/config.go

@@ -32,6 +32,12 @@ const (
 	filePermissions = 0o600
 )
 
+// errHostDataDirNotAbsolute is returned when UPAAS_HOST_DATA_DIR is set to a
+// relative path, which the Docker daemon cannot resolve for app bind mounts.
+var errHostDataDirNotAbsolute = errors.New(
+	"UPAAS_HOST_DATA_DIR must be an absolute path",
+)
+
 // Params contains dependencies for Config.
 type Params struct {
 	fx.In
@@ -124,6 +130,10 @@ func buildConfig(log *slog.Logger, params *Params) (*Config, error) {
 	dataDir := viper.GetString("DATA_DIR")
 	hostDataDir := viper.GetString("HOST_DATA_DIR")
 
+	if hostDataDir != "" && !filepath.IsAbs(hostDataDir) {
+		return nil, fmt.Errorf("%w, got %q", errHostDataDirNotAbsolute, hostDataDir)
+	}
+
 	if hostDataDir == "" {
 		hostDataDir = dataDir
 	}