[docker] Fix ERLANG_EXITS typo in healthcheck script

[jacky-w-j-li]

The Docker healthcheck script docker/base_image/healthcheck.sh contains a variable name typo: ERLANG_EXITS should be ERLANG_EXIST.

Root cause

Line 35 assigns the variable as ERLANG_EXITS, and line 37 references it as $ERLANG_EXITS. While the script is internally consistent (the same misspelled name is used in both places, so the script works), the name is clearly intended to be ERLANG_EXIST to match the sibling variable KAZOO_EXIST on line 34, and to reflect the actual meaning (checking whether the module exists).

Fix

Rename ERLANG_EXITS to ERLANG_EXIST in both the assignment (line 35) and the conditional check (line 37).

Testing

• Verified the script logic is unchanged: the variable is renamed consistently in both the assignment and the usage.
• No behavioral change — the script was already working because the same (misspelled) name was used throughout.

Resolves: #3126