Dockerize LocalStack MCP Server as a Self-Contained Image

[HarshCasper]

Fixes DEVREL-292

Summary

• add a Docker image (localstack/localstack-mcp-server, built for linux/amd64 and linux/arm64) that bundles the LocalStack CLI, awslocal, Terraform/tflocal, CDK/cdklocal, SAM/samlocal, the Snowflake snow CLI, and the Docker CLI, so Docker is the only thing needed on the host
• start LocalStack as a sibling container through the mounted /var/run/docker.sock; the server and IaC CLIs reach it over host.docker.internal, set with LOCALSTACK_HOSTNAME and the --add-host aliases
• inject the LocalStack endpoint plus credential and region defaults into the IaC CLIs (buildIacCliEnv) only in container mode, including the s3. endpoint and path-style S3 (patched into the bundled aws-cdk) so cdklocal asset uploads work
• leave the npx/host setup unchanged: that env is added only when LOCALSTACK_HOSTNAME is not localhost, so host installs behave as before
• build and test the image on PRs; on main and release tags, push it to Docker Hub (:latest and :sha-<short> from main, :X.Y.Z and :X.Y from tags) through a docker-push environment restricted to those refs, and sync the README to the Docker Hub page
• add a test harness (tests/docker/validate-image.mjs) that drives the image over MCP stdio, including Terraform and CDK deploys against LocalStack, with sample projects under data/; document the setup in README.md and docs/DOCKER.md

[remotesynth]

Love the idea but ran several tests of this and have a couple thoughts/issues:

1. The configuration is pretty complex and I worry that may be a blocker to usage. I'm not sure if there is a way to simplify this, but it's very easy to mess up (I did multiple times before getting it right).
2. I couldn't get it to deploy a CDK app. It says Deployer failed because cdklocal isn't installed. Checking prerequisites and installing it.. It then tried to go through a whole series of install processes (which I stopped).