Add a GNU Guix manifest (#8038)

## Testing

See https://codeberg.org/forgejo/docs/pulls/1224

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8038
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Co-committed-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>

manifest.scm

@@ -0,0 +1,38 @@
+;;; Copyright 2025 The Forgejo Authors. All rights reserved.
+;;; SPDX-License-Identifier: MIT
+;;;
+;;; Commentary:
+;;;
+;;; This is a GNU Guix manifest that can be used to create a
+;;; development environment to build and test Forgejo.
+;;;
+;;; The following is a usage example to create a containerized
+;;; environment, with HOME shared for the Go cache and the network
+;;; made available to fetch required Go and Node dependencies.
+;;;
+#|
+guix shell -CNF --share=$HOME -m manifest.scm
+export GOTOOLCHAIN=local     # to use the Go binary from Guix
+export CC=gcc CGO_ENABLED=1
+export TAGS="timetzdata sqlite sqlite_unlock_notify"
+make clean
+make -j$(nproc)
+make test -j$(nproc)         # run unit tests
+make test-sqlite -j$(nproc)  # run integration tests
+make watch                   # run an instance/rebuild on changes
+|#
+(specifications->manifest
+ (list "bash-minimal"
+       "coreutils"
+       "findutils"
+       "gcc-toolchain"
+       "git"                            ;libpcre support is required
+       "git-lfs"
+       "gnupg"
+       "go"
+       "grep"
+       "make"
+       "node"
+       "nss-certs"
+       "openssh"
+       "sed"))