Run Docker container as current user (#1975)

Previously, mega-linter-runner ran the MegaLinter Docker image as root.
Users whose files became owned by root as a consequence of this
behavior will need to chown them to be owned by the appropriate user.
This change only affects POSIX platforms, because process.getuid and
process.getgid are only available there.

Author: Kurt-von-Laven

CHANGELOG.md

@@ -19,6 +19,8 @@ Note: Can be used with `oxsecurity/megalinter@beta` in your GitHub Action mega-l
     by @Kurt-von-Laven in [#2275](https://github.com/oxsecurity/megalinter/pull/2275).
   - Only run npm-package-json-lint when package.json is present by @Kurt-von-Laven in
     [#2280](https://github.com/oxsecurity/megalinter/pull/2280).
+  - Run Docker container as current user rather than root by @Kurt-von-Laven in
+    [#1975](https://github.com/oxsecurity/megalinter/issues/1975).
 
 - Documentation
   - Configure jsonschema documentation formatting (see [Descriptor schema](https://megalinter.io/latest/json-schemas/descriptor.html), [Configuration schema](https://megalinter.io/latest/json-schemas/configuration.html)), by @echoix in [#2270](https://github.com/oxsecurity/megalinter/pull/2270)

mega-linter-runner/lib/runner.js

@@ -4,6 +4,7 @@ const optionsDefinition = require("./options");
 const { spawnSync } = require("child_process");
 const c = require("chalk");
 const path = require("path");
+const { getgid, getuid } = require("process");
 const which = require("which");
 const fs = require("fs-extra");
 const { MegaLinterUpgrader } = require("./upgrade");
@@ -127,6 +128,9 @@ ERROR: Docker engine has not been found on your system.
     if (options["containerName"]) {
       commandArgs.push(...["--name", options["containerName"]]);
     }
+    if (getuid && getgid) {
+      commandArgs.push(...["--user", `${getuid()}:${getgid()}`]);
+    }
     commandArgs.push(...["-v", "/var/run/docker.sock:/var/run/docker.sock:rw"]);
     commandArgs.push(...["-v", `${lintPath}:/tmp/lint:rw`]);
     if (options.fix === true) {