dockerTools: document environment helpers

doc/builders/images/dockertools.section.md

@@ -292,7 +292,44 @@ The parameters relative to the base image have the same synopsis as described in
 
 The `name` argument is the name of the derivation output, which defaults to `fromImage.name`.
 
-## shadowSetup {#ssec-pkgs-dockerTools-shadowSetup}
+## Environment Helpers {#ssec-pkgs-dockerTools-helpers}
+
+Some packages expect certain files to be available globally.
+When building an image from scratch (i.e. without `fromImage`), these files are missing.
+`pkgs.dockerTools` provides some helpers to set up an environment with the necessary files.
+You can include them in `copyToRoot` like this:
+
+```nix
+buildImage {
+  name = "environment-example";
+  copyToRoot = with pkgs.dockerTools; [
+    usrBinEnv
+    binSh
+    caCertificates
+    fakeNss
+  ];
+}
+```
+
+### usrBinEnv {#sssec-pkgs-dockerTools-helpers-usrBinEnv}
+
+This provides the `env` utility at `/usr/bin/env`.
+
+### binSh {#sssec-pkgs-dockerTools-helpers-binSh}
+
+This provides `bashInteractive` at `/bin/sh`.
+
+### caCertificates {#sssec-pkgs-dockerTools-helpers-caCertificates}
+
+This sets up `/etc/ssl/certs/ca-certificates.crt`.
+
+### fakeNss {#sssec-pkgs-dockerTools-helpers-fakeNss}
+
+Provides `/etc/passwd` and `/etc/group` that contain root and nobody.
+Useful when packaging binaries that insist on using nss to look up
+username/groups (like nginx).
+
+### shadowSetup {#ssec-pkgs-dockerTools-shadowSetup}
 
 This constant string is a helper for setting up the base files for managing users and groups, only if such files don't exist already. It is suitable for being used in a [`buildImage` `runAsRoot`](#ex-dockerTools-buildImage-runAsRoot) script for cases like in the example below: