feat: Update devcontainer.json + add Dockerfile (leanprover-community#2998)

This PR removes the external dependency for `devcontainer.json` by using a dockerfile to install elan.

Another feature is that `lake exe cache get!` is now done using `onCreateCommand` instead of `postCreateCommand`, so the oleans should be available as soon as the container is created, whereas before the user had to wait a little while, which was potentially confusing.

Author: adamtopaz

.devcontainer/Dockerfile

@@ -0,0 +1,8 @@
+FROM mcr.microsoft.com/devcontainers/base:jammy
+
+USER vscode
+WORKDIR /home/vscode
+
+RUN curl https://raw.githubusercontent.com/leanprover/elan/master/elan-init.sh -sSf | sh -s -- -y --default-toolchain none
+
+ENV PATH="/home/vscode/.elan/bin:${PATH}"

.devcontainer/devcontainer.json

@@ -1,10 +1,12 @@
 {
-  "name": "Ubuntu",
-  "image": "mcr.microsoft.com/devcontainers/base:jammy",
-  "features": {
-    "ghcr.io/adamtopaz/elan-feature/elan:0" : { "version" : "0.1.1" }
+  "name": "Mathlib4 dev container",
+
+  "build": {
+    "dockerfile": "Dockerfile"
   },
-  "postCreateCommand": "lake exe cache get!",
+
+  "onCreateCommand": "lake exe cache get!",
+
   "customizations": {
     "vscode" : {
       "extensions" : [ "leanprover.lean4" ]