[service] [request]: Custom seccomp profile for ECS containers

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**Tell us about your request**
Ability to add custom seccomp profile to ECS containers

**Which service(s) is this request for?**
ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We'd like to securely run headless chromium on ECS. We'd like to run the container giving the least privileges possible and that involves passing in a custom seccomp.json file (https://github.com/docker/for-linux/issues/496#issuecomment-441149510).

ECS `dockerSecurityOptions` doesn't not support custom seccomp files.

**Are you currently working around this issue?**
We are using EKS as a workaround and this is blocking our migration to ECS.

**Additional context**
Anything else we should know?

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Start by reviewing the ECS dockerSecurityOptions limitation and the linked Docker seccomp discussion. The request is complete when ECS containers can receive a custom seccomp.json profile with the least required privileges, enabling the headless Chromium use case without the EKS workaround.

Written by the indexing model from the issue text.