Pre-install server deps for fast devcontainer start. Fix dev launch.json

There seems to be a more standard mechanism of specifying launch.json
params for devcontainers. Previous mechanism to write launch.json to
.vscode/launch.json in post creation step does not work.

Improve default launch.json to include khoj admin username, password
with placeholder values to get started with local development faster.

Define dockerfile for devcontainer to pre-built server, web app
dependencies during dev container image creation stage. So install on
dev container startup is sped up as no need to install dependencies.

.devcontainer/devcontainer.json

@@ -1,6 +1,12 @@
 {
     "name": "Khoj Development Environment",
-    "image": "mcr.microsoft.com/devcontainers/python:3.10",
+    "build": {
+        "dockerfile": "Dockerfile",
+        "context": "..", // Build context is the project root
+        "args": {
+            "PYTHON_VERSION": "3.10"
+        }
+    },
     "forwardPorts": [
         42110
     ],
@@ -40,11 +46,39 @@
                     "tests"
                 ],
                 "python.testing.unittestEnabled": false,
-                "python.testing.pytestEnabled": true
+                "python.testing.pytestEnabled": true,
+                "launch": {
+                    "configurations": [
+                        {
+                            "name": "Launch Khoj",
+                            "type": "debugpy",
+                            "request": "launch",
+                            "module": "src.khoj.main",
+                            "console": "integratedTerminal",
+                            "justMyCode": false,
+                            "sudo": true,
+                            "args": [
+                                "-v",
+                                "--anonymous-mode",
+                                "--non-interactive",
+                                "--port=42110"
+                            ],
+                            "envFile": "${workspaceFolder}/.env",
+                            "env": {
+                                "KHOJ_ADMIN_EMAIL": "admin",
+                                "KHOJ_ADMIN_PASSWORD": "admin",
+                                "USE_EMBEDDED_DB": "true",
+                                "KHOJ_DEBUG": "true",
+                                "KHOJ_TELEMETRY_DISABLED": "true",
+                                "PROMPTRACE_DIR": "${workspaceFolder}/promptrace"
+                            }
+                        }
+                    ]
+                }
             }
         }
     },
-    "postCreateCommand": "scripts/dev_setup.sh --devcontainer",
+    "postCreateCommand": "scripts/dev_setup.sh",
     "features": {
         "ghcr.io/devcontainers-contrib/features/black:2": {},
         "ghcr.io/devcontainers-contrib/features/mypy:2": {},