Add DOCKER_FLAGS support for passing extra flags to the emulator container

[anisaoshafi]

Problem

lstk had no way to pass arbitrary Docker flags when starting an emulator if you needed to inject environment variables (-e SERVICES=s3,sqs), or extra volume mounts.

The root cause was structural: runtime.ContainerConfig only modeled the options lstk explicitly needed (port bindings, known env vars, the volume mount for persistence).

Fix

Added support for a DOCKER_FLAGS environment variable and a docker_flags field in the [[containers]] config.toml block.
Both accept a raw docker-run-style string (e.g. "-e SERVICES=s3,sqs -v /tmp/data:/data").
The two sources are merged: env var flags apply globally across all containers, config flags apply per-container.

Supported flags: -e/--env, -v/--volume.

[carole-lavillonniere]

@anisaoshafi I don't fully get why we need this new feature.

• -e SERVICES=s3,sqs: this should be possible using the current config format:

  [env.aws_services]
  SERVICES = "s3,sqs"

  [[containers]]
  type = "aws"
  port = "4566"
  env = ["aws_services"]

• extra volume mounts: not supported by config yet, but it could:

[[containers]]
type = "aws"
volume = "/tmp/data:/data"

could become:

[[containers]]
type = "aws"
volumes = ["/tmp/data:/data", "/home/me/init:/etc/localstack/init/ready.d:ro"]

Am I missing something?

[anisaoshafi]

@carole-lavillonniere that's true, there are workarounds for env vars, though not directly for volumes.

In the docs, we support customer DOCKER_FLAGS, and Maureen was expecting this to work out of the box since it works with the old CLI. I took the liberty to support this, but probably should have asked for more input.
@mmaureenliu what do you think?

[mmaureenliu]

@carole-lavillonniere that's true, there are workarounds for env vars, though not directly for volumes.

In the docs, we support customer DOCKER_FLAGS, and Maureen was expecting this to work out of the box since it works with the old CLI. I took the liberty to support this, but probably should have asked for more input. @mmaureenliu what do you think?

I like Carole's suggestion if it works. My initial intuition was also to look at the volume config but then realised it was only used for persistence.

It would also be much cleaner than DOCKER_FLAGS - current localstack implementation apparently struggles with space in path