Rework of .devcontainer (#1816) (#1817)

> The only issue that I currently have with this one is that the `clangd` extension is having some hiccups, not being able to find includes like `<gmock/gmock.h>` and `<benchmark/benchmark.h>` and thus showing extra errors. However, this only affects the VSCode extension and bazel seems to work fine.

This might be an issue with the compilation database: you might try ./scripts/create_compdb.py to help.

.devcontainer/devcontainer.json

@@ -1,12 +1,21 @@
 {
-  "name": "dev environment",
-  "dockerFile": "Dockerfile",
-  "extensions": [
-    "bazelbuild.vscode-bazel",
-    "bierner.github-markdown-preview",
-    "daohong-emilio.yash",
-    "esbenp.prettier-vscode",
-    "llvm-vs-code-extensions.vscode-clangd",
-    "ms-python.python"
-  ]
+  "name": "carbon-lang",
+  "build": {
+    "dockerfile": "Dockerfile",
+    "args": {
+      "TAG": "3.5.6"
+    }
+  },
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "bazelbuild.vscode-bazel",
+        "bierner.github-markdown-preview",
+        "daohong-emilio.yash",
+        "esbenp.prettier-vscode",
+        "llvm-vs-code-extensions.vscode-clangd",
+        "ms-python.python"
+      ]
+    }
+  }
 }