self-hosted cymbal container failed to connect redis

Bug Description

Hello, to solve the problem of error tracking on self-hosted (https://github.com/posthog/posthog/issues/38918), I tried to start the cymbal container with this PR(https://github.com/PostHog/posthog/pull/41841).
The cymbal image's hash version is ghcr.io/posthog/posthog/cymbal@sha256:7f1afe72268f150310a303bdfc8f17db27709fcd134d12b1de8a8391ef3c67ef
(extracted from the following output:

dev@poc-vm-02:~/posthog-new/posthog-master$ docker image inspect 9caca62a58e4
[
    {
        "Id": "sha256:9caca62a58e42dd7566417120aec90e70a683f3183684c2adaa2cfd3ffb54916",
        "RepoTags": [
            "ghcr.io/posthog/posthog/cymbal:master"
        ],
        "RepoDigests": [
            "ghcr.io/posthog/posthog/cymbal@sha256:7f1afe72268f150310a303bdfc8f17db27709fcd134d12b1de8a8391ef3c67ef"
        ],
        "Parent": "",
        "Comment": "buildkit.dockerfile.v0",
        "Created": "2026-02-09T18:01:44.135893801Z",
        "DockerVersion": "",
        "Author": "",
......

)

But when the cymbal container failed to start with the following error:

2026-02-11T02:20:52.842359Z  INFO cymbal: Starting up...
2026-02-11T02:20:52.842449Z  INFO common_continuous_profiling: Continuous profiling is disabled
2026-02-11T02:20:52.842465Z  WARN cymbal: Posthog client disabled
2026-02-11T02:20:52.851722Z  WARN cymbal::config: Falling back to building config from explicit environment variables

thread 'main' (7) panicked at cymbal/src/main.rs:57:67:
called `Result::unwrap()` on an `Err` value: RedisError(Redis(Connection refused (os error 111)))
stack backtrace:
   0:     0x5630c93debf2 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::haa87a551a4affa55
   1:     0x5630c93f284f - core::fmt::write::h80461e1e45e4fdd2
   2:     0x5630c93ab0e1 - std::io::Write::write_fmt::h6e6c69b2d6337d9b
   3:     0x5630c93b7a42 - std::sys::backtrace::BacktraceLock::print::hf67a46baa621998e
   4:     0x5630c93bd0ff - std::panicking::default_hook::{{closure}}::h391aa815d5e47ec8
   5:     0x5630c93bcf59 - std::panicking::default_hook::hd6fdcf2489bb807d
   6:     0x5630c93bd835 - std::panicking::panic_with_hook::h185ddfb86bf14d73
   7:     0x5630c93bd61a - std::panicking::panic_handler::{{closure}}::had89ddd01b6112c9
   8:     0x5630c93b7b79 - std::sys::backtrace::__rust_end_short_backtrace::h5d0fc36eef7265ea
   9:     0x5630c939e62d - __rustc[eb8946e36839644a]::rust_begin_unwind
  10:     0x5630c93fdd50 - core::panicking::panic_fmt::h92c8e5abe71dd8d1
  11:     0x5630c93fcd56 - core::result::unwrap_failed::h0501379eaec3e720
  12:     0x5630c799c22b - cymbal::main::{{closure}}::ha261d8a87d1404ed
  13:     0x5630c7991b84 - tokio::runtime::park::CachedParkThread::block_on::h3c9b65ff98ab6ae1
  14:     0x5630c7729cec - tokio::runtime::runtime::Runtime::block_on::h1736034a67f4e933
  15:     0x5630c77dae52 - cymbal::main::h3f587d706d633b2d
  16:     0x5630c7945353 - std::sys::backtrace::__rust_begin_short_backtrace::hcf6f41e831909d84
  17:     0x5630c7879579 - std::rt::lang_start::{{closure}}::h92179c0834e0ea86
  18:     0x5630c93acc10 - std::rt::lang_start_internal::h6ba36b077a531782
  19:     0x5630c77e6b55 - main
  20:     0x7f2a128a724a - <unknown>
  21:     0x7f2a128a7305 - __libc_start_main
  22:     0x5630c76ad121 - _start
  23:                0x0 - <unknown>

And the following is how I configure the redis in env variables:

  cymbal:
       image: ghcr.io/posthog/posthog/cymbal:master
       build:
           context: rust/
           args:
               BIN: cymbal
       restart: on-failure
       environment:
           REDIS_URL: "redis://109.144.5.242:6381"
           PERSONS_URL: "postgres://posthog:posthog@db:5432/posthog"
           DATABASE_URL: "postgres://posthog:posthog@db:5432/posthog"  #'postgres://posthog:posthog@109.144.6.162:5432/posthog' #'postgres://posthog:posthog@db:5432/posthog'
           KAFKA_HOSTS: 'kafka:9092'
           #RUST_BACKTRACE: "1"
           RUST_BACKTRACE: "full"
       depends_on:
           db:
               condition: service_healthy
           kafka:
               condition: service_started

There is no error with redis because I can connect it as follows:

I have no name!@redis-cluster-0:/$ redis-cli -u redis://109.144.5.242:6381
109.144.5.242:6381> dbsize
(integer) 1506
109.144.5.242:6381> 

Is there anything I can do to solve this? thanks!

Debug info

- [√ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [build from https://github.com/PostHog/posthog/commit/7866528acd998bbb51c8afcc24749aeea0a06e2d]

Start at cymbal/src/main.rs:57 and review the cymbal service environment in the shown Docker Compose configuration, especially REDIS_URL. Compare the container's Redis connection context with the successful redis-cli test. Done means cymbal starts without panicking and connects to Redis successfully.