Possibility to use -u (user) option in docker exec (#203)

* Add user option for docker exec

* Add test for user option

* Return test version for image

* remove gitea config file

* refactor tests

* remove comments & fix image name

* add docs

* cleanup

* Update README.md with suggested correction

Co-authored-by: Frederik Ring <frederik.ring@gmail.com>

* fix backup command & bind folder instead of volume

---------

Co-authored-by: tao <generaltao.md@gmail.com>
Co-authored-by: Frederik Ring <frederik.ring@gmail.com>

test/user/run.sh

@@ -0,0 +1,30 @@
+#!/bin/sh
+
+set -e
+
+cd $(dirname $0)
+. ../util.sh
+current_test=$(basename $(pwd))
+
+docker compose up -d
+
+user_name=testuser
+docker exec user-alpine-1 adduser --disabled-password "$user_name"
+
+docker compose exec backup backup
+
+tar -xvf ./local/test.tar.gz
+if [ ! -f ./backup/data/whoami.txt ]; then
+  fail "Could not find file written by pre command."
+fi
+pass "Found expected file."
+
+tar -xvf ./local/test.tar.gz
+if [ "$(cat ./backup/data/whoami.txt)" != "$user_name" ]; then
+  fail "Could not find expected user name."
+fi
+pass "Found expected user."
+
+docker compose down --volumes
+sudo rm -rf ./local
+