fix: ensure setting /github/workdir as Git safe directory works always (#6242)

When running super-linter on in a self hosted runner based on Action
Runner Controller in K8s container mode the /github/home directory is
not neccesarily mounted. But since we expect this directory to exist
super-linter fails to set /github/workspace as a Git safe directory:

    2024-10-04 17:58:37 [INFO]   Successfully validated GITHUB_WORKSPACE: /github/workspace
    2024-10-04 17:58:37 [DEBUG]   Configuring Git safe directories
    2024-10-04 17:58:37 [DEBUG]   Set /github/workspace as a Git safe directory
    error: could not lock config file /github/home/.gitconfig: No such file or directory
    2024-10-04 17:58:37 [FATAL]   Cannot configure /github/workspace as a Git safe directory.

Making sure this directory exists in container image ensures that under
no circumstances we can fail - if the action runner provides
/github/home it will be mounted over, and if not we provide a directory
where git can actually create the new .gitconfig.

This also defaults the HOME environment variable to /github/home as it
is the default in most places. So if somebody decides to override HOME
they are also expected to ensure that this directory exists (or is a
volume mount)

Dockerfile

@@ -434,6 +434,11 @@ ENV RENOVATE_X_IGNORE_RE2="true"
 ENV VERSION_FILE="/action/linterVersions.txt"
 RUN mkdir /action
 
+# create the homedir, so that in case it is not present (like on action-runner-controller based selfhosted runners)
+# we do not fail at setting /github/workspace as a safe git directory
+ENV HOME="/github/home"
+RUN mkdir -p "${HOME}"
+
 ENTRYPOINT ["/action/lib/linter.sh"]
 
 FROM base_image AS slim

test/inspec/super-linter/controls/super_linter.rb

@@ -18,6 +18,10 @@ control "super-linter-environment-variables" do
     its("content") { should match(/^(standard|slim)$/) }
   end
 
+  describe os_env("HOME") do
+    its("content") { should eq "/github/home" }
+  end
+
   describe os_env("RENOVATE_X_IGNORE_RE2") do
     its("content") { should eq "true" }
   end
@@ -437,6 +441,7 @@ control "super-linter-validate-directories" do
   desc "Check that directories that Super-Linter needs are installed."
 
   dirs = [
+    "/github/home",
     "/node_modules",
     "/action/lib",
     "/action/lib/functions",