Profile [env.*] cannot override LOCALSTACK_HOST (launcher value appended last)

从 internal/container/start.go 中环境变量组装部分附近的第266–296行开始；比较 SF_S3_ENDPOINT 的检查和针对 MAIN_CONTAINER_NAME 的警告，以及 LOCALSTACK_HOST 的追加方式。检查附近有关容器环境构造的测试，然后为 profile 提供的 LOCALSTACK_HOST 添加覆盖。完成标准是：要么遵循 profile 值，要么给出清晰的警告说明该值被忽略，并且所选行为经过测试。

Observed

Setting LOCALSTACK_HOST in a config [env.*] profile has no effect. The container receives the variable twice — the profile value first, lstk's computed value second — and Docker's last-wins semantics keep lstk's. docker inspect on a type = "snowflake" container started with a profile containing LOCALSTACK_HOST = "sf.example.test:4599" shows both entries; the emulator sees localhost.localstack.cloud:4599. Nothing warns that the profile value was dropped.

Where

internal/container/start.go on main (d9c83037):

• :276 env := resolvedEnv — the profile's variables come first;
• :281-285 env = append(env, "GATEWAY_LISTEN=…", "MAIN_CONTAINER_NAME=…", "LOCALSTACK_HOST="+endpoint.Hostname+":"+c.Port) — lstk's own value is appended unconditionally after them.

The two neighbours already handle this collision: SF_S3_ENDPOINT (:292) is appended only when the profile did not set it (!envHasKey(resolvedEnv, "SF_S3_ENDPOINT")), and MAIN_CONTAINER_NAME (:266-275) emits a warning saying the profile value is ignored. LOCALSTACK_HOST does neither, so the profile value vanishes silently. (A LOCALSTACK_HOST exported in the host shell does win, because hostEnv is appended at :296 after lstk's line — so the two configuration channels behave differently.)

Context

Recorded in September 2026 while diagnosing a snowflake-next result-chunk 404 under lstk (the emulator advertised the host lstk injected but its hostname gate did not route it; that was fixed on the emulator side, which now routes any host it is told to advertise). For the default flow this is harmless: the value lstk injects, localhost.localstack.cloud:<published port>, is the right one. It only matters when a user needs a different advertised host — a non-loopback hostname for clients on other machines, or a reverse-proxy name — and reaches for the documented [env.*] mechanism to set it.

Expected

Either the profile wins — append lstk's LOCALSTACK_HOST only when the profile did not set it, with the same envHasKey guard SF_S3_ENDPOINT uses — or lstk warns that the profile value is ignored, as it does for MAIN_CONTAINER_NAME, and the config docs state that LOCALSTACK_HOST is launcher-owned.

Reproduce

[[containers]]
type = "snowflake"
port = "4599"
env = ["custom"]

[env.custom]
LOCALSTACK_HOST = "sf.example.test:4599"

lstk start, then docker inspect <container> --format '{{json .Config.Env}}' → both LOCALSTACK_HOST=sf.example.test:4599 and LOCALSTACK_HOST=localhost.localstack.cloud:4599, in that order; inside the container echo $LOCALSTACK_HOST prints the latter.