chore: Add support for launching a Gitpod by specifying build image (#1127)

Author: marcusroberts

.gitpod.Dockerfile

@@ -0,0 +1,3 @@
+FROM gitpod/workspace-full
+
+ENV PATH="/workspace/grain/node_modules/.bin:$PATH"

.gitpod.yml

@@ -0,0 +1,15 @@
+# This configuration file was automatically generated by Gitpod.
+# Please adjust to your needs (see https://www.gitpod.io/docs/config-gitpod-file)
+# and commit this file to your remote git repository to share the goodness with others.
+
+image:
+  file: .gitpod.Dockerfile
+
+tasks:
+  - init: |
+      yarn --pure-lockfile
+      yarn compiler build
+vscode:
+  extensions:
+    - ocamllabs.ocaml-platform
+    - grain-lang.vscode-grain