### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

There appears to be an issue with this variable `export LAMBDA_KEEPALIVE_MS=1337`.
It's not picked up when you start `localstack start -d`

```shell
$ export LAMBDA_KEEPALIVE_MS=1000
$ env | grep LAMBDA_KEEPALIVE_MS
LAMBDA_KEEPALIVE_MS=1000
$ localstack start -d
$ docker inspect localstack_main -f '{{json .Config.Env}}' | grep LAMBDA_KEEPALIVE_MS
```

### Expected Behavior

It should be set in the container environment.

### How are you starting LocalStack?

With the `localstack` script

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

    docker run localstack/localstack

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

    awslocal s3 mb s3://mybucket


### Environment

```markdown
- OS: Windows (WSL)
- LocalStack: 2.2.1.dev20230731233910
```


### Anything else?

_No response_
TRACE_LOG_LEVELS,
    TRUE_STRINGS,
)
T = TypeVar("T", str, int)
load_start_time = time.time()
class Directories:
    """
    Holds the different directories available to localstack. Some directories are shared between the host and the
    localstack container, some live only on the host and some only in the container.
    Attributes:
        static_libs: container only; binaries and libraries statically packaged with the image
        var_libs:    shared; binaries and libraries+data computed at runtime: lazy-loaded binaries, ssl cert, ...
        cache:       shared; ephemeral data that has to persist across localstack runs and reboots
        tmp:         shared; ephemeral data that has to persist across localstack runs but not reboots
        functions:   shared; volume to communicate between host<->lambda containers
        data:        shared; holds localstack state, pods, ...
        config:      host only; pre-defined configuration values, cached credentials, machine id, ...
        init:        shared; user-defined provisioning scripts executed in the container when it starts
        logs:        shared; log files produced by localstack
    """
    static_libs: str
    var_libs: str
    cache: str
    tmp: str
    functions: str
    data: str
    config: str
    init: str
    logs: str
    def __init__(