switch to miniconda3 base image, add Java via SDKMAN

- Base image: continuumio/miniconda3 (conda + python 3 out of the box)
- Java: installed via SDKMAN (sdk install java) — latest LTS by default
- Node.js: nvm install kept from previous change
- All tool envs (nvm, sdkman) sourced before exec so Python and all
  child processes (exec-tool, jobs) inherit the full PATH

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   mcp-bash:
-    image: python:3.12-slim
+    image: continuumio/miniconda3:latest
     container_name: mcp-bash
     restart: unless-stopped
     working_dir: /workspace