environment: Provide /etc/hosts in containers without '--network'.

Fixes <https://bugs.gnu.org/43762>.

* guix/scripts/environment.scm (launch-environment/container): Create
/etc/hosts when NETWORK? is false.
* tests/guix-environment-container.sh: Add "localhost" resolution test.

guix/scripts/environment.scm

@@ -549,6 +549,13 @@ WHILE-LIST."
             (write-passwd (list passwd))
             (write-group groups)
 
+            (unless network?
+              ;; When isolated from the network, provide a minimal /etc/hosts
+              ;; to resolve "localhost".
+              (call-with-output-file "/etc/hosts"
+                (lambda (port)
+                  (display "127.0.0.1 localhost\n" port))))
+
             ;; For convenience, start in the user's current working
             ;; directory or, if unmapped, the home directory.
             (chdir (if map-cwd?

tests/guix-environment-container.sh

@@ -44,6 +44,10 @@ else
     test $? = 42
 fi
 
+# Make sure "localhost" resolves.
+guix environment --container --ad-hoc --bootstrap guile-bootstrap \
+     -- guile -c '(exit (pair? (getaddrinfo "localhost" "80")))'
+
 # Make sure '--preserve' is honored.
 result="`FOOBAR=42; export FOOBAR; guix environment -C --ad-hoc --bootstrap \
    guile-bootstrap -E ^FOO -- guile -c '(display (getenv \"FOOBAR\"))'`"