client/container: implement debug log-levels

DOCKER_FINANCE_DEBUG will now support the following:

0 = no debug output
1 = `print_debug` (bash/PHP)
2 = level 1 + `set -xv` (bash)

client/docker-finance.d/client/env/gen.bash

@@ -31,8 +31,8 @@ export DOCKER_FINANCE_VERSION="$global_client_version"
 
 # Developer related
 
-if [ -z "$DOCKER_FINANCE_DEBUG" ]; then
-  export DOCKER_FINANCE_DEBUG=false
+if [[ -z "$DOCKER_FINANCE_DEBUG" || ! "$DOCKER_FINANCE_DEBUG" =~ ^0$|^1$|^2$ ]]; then
+  export DOCKER_FINANCE_DEBUG=0
 fi
 
 # Allows transparent r/w of mounted volumes