[BUG] --local-build fails even though docker is installed

**Describe the bug**
Docker is installed but --local-build fails.

**To Reproduce**
```
agentcore launch --local-build
🔧 Launching Bedrock AgentCore (local-build mode - NEW!)...
• Build container locally with Docker
• Deploy to Bedrock AgentCore cloud runtime
• Requires Docker/Finch/Podman to be installed
• Use when you need custom build control

Using existing memory: ac_pg_mem-5sxtss21Ir
Using existing memory: ac_pg_mem-5sxtss21Ir
Launching Bedrock AgentCore agent 'ac_pg' to cloud
❌ No supported container engine found.

AgentCore requires one of the following container engines for local builds:
• Docker (any recent version, including Docker Desktop)
• Finch (Amazon's open-source container engine)
• Podman (compatible alternative to Docker)

To install:
• Docker: https://docs.docker.com/get-docker/
• Finch: https://github.com/runfinch/finch
• Podman: https://podman.io/getting-started/installation

Alternative: Use CodeBuild for cloud-based building (no container engine needed):
agentcore launch # Uses CodeBuild (default)

which docker
/usr/bin/docker

docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
```

**Expected behavior**
Is there a way to specify how to find docker?

**Error Output**
```
Paste any error messages or stack traces here
```

**Environment:**
uname -a
Linux ip-10-0-3-188 6.14.0-1011-aws #11~24.04.1-Ubuntu SMP Fri Aug 1 02:07:25 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
python --version
Python 3.12.3
(installed with pip in venv)
agentcore status
✅ MemoryManager initialized for region: us-west-2
🔎 Retrieving memory resource with ID: ac_pg_mem-5sxtss21Ir...
Found memory: ac_pg_mem-5sxtss21Ir
╭─────────────────────────────────────────────────────────────────────── Agent Status: ac_pg ────────────────────────────────────────────────────────────────────────╮
│ ⚠️ Configured but not deployed │
│ │
│ Agent Details: │
│ Agent Name: ac_pg │
│ Region: us-west-2 │
│ Account: XXXXX │
│ │
│ Configuration: │
│ Execution Role: arn:aws:iam::125943838787:role/AmazonBedrockAgentCoreSDKRuntime-us-west-2-XXXXX │
│ ECR Repository: 125943838787.dkr.ecr.us-west-2.amazonaws.com/bedrock-agentcore-ac_pg │
│ │
│ Your agent is configured but not yet launched. │
│ │
│ Next Steps: │
│ agentcore launch │
╰────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
**Additional context**
Add any other context about the problem here.

Start by reproducing `agentcore launch --local-build` and compare its container-engine detection with `which docker` and `docker ps`. Trace the implementation behind the `--local-build` entry point; done means an installed Docker at `/usr/bin/docker` is recognized, or the supported way to specify its location is documented and works.

Written by the indexing model from the issue text.