Inject $WORKSPACE_DIR into container

This permits users to check ZMK out into arbitrarily-named directories

.devcontainer/.bashrc

@@ -1,6 +1,6 @@
 export LS_OPTIONS='-F --color=auto'
 eval "`dircolors`"
 alias ls='ls $LS_OPTIONS'
-if [ -f /workspaces/zmk/zephyr/zephyr-env.sh ]; then
-  source /workspaces/zmk/zephyr/zephyr-env.sh
+if [ -f "$WORKSPACE_DIR/zephyr/zephyr-env.sh" ]; then
+  source "$WORKSPACE_DIR/zephyr/zephyr-env.sh"
 fi

.devcontainer/devcontainer.json

@@ -2,6 +2,7 @@
   "name": "ZMK Development",
   "dockerFile": "Dockerfile",
   "extensions": ["ms-vscode.cpptools"],
-  "runArgs": ["--security-opt", "label=disable"]
+  "runArgs": ["--security-opt", "label=disable"],
+  "containerEnv": {"WORKSPACE_DIR": "${containerWorkspaceFolder}"}
 }