Fix Dev Container KBN_DIR (#195810)

## Summary

In #193488, `KBN_DIR` was changed to be a dynamic variable. It wasn't
being properly propagated through the build process in the Dockerfile
and the full path to `env.sh` wasn't being set. This passes the
directory as a build `ARG` as well to fix the path.

.devcontainer/Dockerfile

@@ -1,5 +1,7 @@
 FROM mcr.microsoft.com/devcontainers/base:ubuntu-22.04
 
+ARG KBN_DIR
+
 ENV LANG=en_US.UTF-8 LANGUAGE=en_US:en LC_ALL=en_US.UTF-8
 ENV HOME=/home/vscode
 ENV NVM_DIR=${HOME}/nvm
@@ -67,8 +69,8 @@ RUN mkdir -p $NVM_DIR && \
 USER root
 
 # Reload the env everytime a new shell is opened incase the .env file changed.
-RUN echo "source $KBN_DIR/.devcontainer/scripts/env.sh" >> ${HOME}/.bashrc && \
-  echo "source $KBN_DIR/.devcontainer/scripts/env.sh" >> ${HOME}/.zshrc
+RUN echo "source ${KBN_DIR}/.devcontainer/scripts/env.sh" >> ${HOME}/.bashrc && \
+  echo "source ${KBN_DIR}/.devcontainer/scripts/env.sh" >> ${HOME}/.zshrc
 
 # This is for documentation. Ports are exposed via devcontainer.json
 EXPOSE 9200 5601 9229 9230 9231

.devcontainer/devcontainer.json

@@ -2,7 +2,10 @@
   "name": "Kibana",
   "build": {
     "dockerfile": "Dockerfile",
-    "context": ".."
+    "context": "..",
+    "args": {
+      "KBN_DIR": "${containerWorkspaceFolder}"
+    }
   },
   "customizations": {
     "vscode": {