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

@@ -10,6 +10,7 @@ Note: Can be used with `oxsecurity/megalinter@beta` in your GitHub Action mega-l
 
 - New [cupcake flavor](https://oxsecurity.github.io/megalinter/beta/flavors/cupcake/#readme) with 78 instead of 108 linters
 - Don't add literal double quote character to filenames in mega-linter-runner ([#1942](https://github.com/oxsecurity/megalinter/issues/1942)).
+- Run Docker container as current user rather than root ([#1975](https://github.com/oxsecurity/megalinter/issues/1975))
 - Remove default npm-groovy-lint extra arguments ([#1872](https://github.com/oxsecurity/megalinter/issues/1872))
 
 - Linter versions upgrades

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) {