Use Docker image files in dev container config

See opensafely-core/research-template-docker#32.

In opensafely-core/research-template-docker#57, we moved this
configuration to the Docker image.

We can now update the dev container configuration here to use those
Docker image files.

Note that `postAttachCommand` now becomes a string, instead of an
object; the object format allows for running multiple commands in
parallel:

https://containers.dev/implementors/json_reference/#formatting-string-vs-array-properties

However, for our use case, this is fine for now as we only run
one simple script (that only starts R Studio currently).

.devcontainer/devcontainer.json

@@ -7,10 +7,8 @@
     "features": {
         "ghcr.io/devcontainers/features/docker-in-docker:2": {}
     },
-    "postCreateCommand": "/bin/bash .devcontainer/postCreate.sh ${containerWorkspaceFolder}",
-    "postAttachCommand": {
-        "rstudio-start": "sudo rstudio-server start"
-    },
+    "postCreateCommand": "/bin/bash /opt/devcontainer/postCreate.sh ${containerWorkspaceFolder}",
+    "postAttachCommand": "/bin/bash /opt/devcontainer/postAttach.sh",
     "forwardPorts": [
         8787
     ],