Fix macOS compatibility: replace Bash 4 associative array with string tracking

macOS ships with Bash 3.2 which doesn't support 'declare -A' (associative arrays).
Replace the 'seen' associative array with a simple delimited string 'seen_keys'
to track which environment variables have been processed.

The logic remains identical:
- String concatenation instead of array assignment
- Substring match instead of array key lookup

Tested on macOS with Bash 3.2.57 and the upsert_env function works correctly.

docker-setup.sh

@@ -124,7 +124,7 @@ upsert_env() {
   local -a keys=("$@")
   local tmp
   tmp="$(mktemp)"
-  declare -A seen=()
+  local seen_keys=""
 
   if [[ -f "$file" ]]; then
     while IFS= read -r line || [[ -n "$line" ]]; do
@@ -133,7 +133,7 @@ upsert_env() {
       for k in "${keys[@]}"; do
         if [[ "$key" == "$k" ]]; then
           printf '%s=%s\n' "$k" "${!k-}" >>"$tmp"
-          seen["$k"]=1
+          seen_keys="${seen_keys}${k};"
           replaced=true
           break
         fi
@@ -145,7 +145,7 @@ upsert_env() {
   fi
 
   for k in "${keys[@]}"; do
-    if [[ -z "${seen[$k]:-}" ]]; then
+    if [[ "$seen_keys" != *"${k};"* ]]; then
       printf '%s=%s\n' "$k" "${!k-}" >>"$tmp"
     fi
   done