Install SSH and GPG into container

Brings back the dockerfile so we can install SSH and GPG into it.
This fixes git actions with SSH remotes, and GPG signing now
works out of the box.

This commit was made from inside the container!

.devcontainer/devcontainer.json

@@ -1,6 +1,6 @@
 {
   "name": "ZMK Development",
-  "image": "zmkfirmware/zephyr-west-action-arm",
+  "dockerFile": "Dockerfile",
   "extensions": ["ms-vscode.cpptools"],
   "runArgs": ["--security-opt", "label=disable"]
 }