Address .devcontainer/boot.sh: 3: [[: not found

This commit addresses the following `.devcontainer/boot.sh: 3: [[: not found`
`[[` and `]]` syntax supported by Bash is not available /bin/bash

https://www.gnu.org/software/bash/manual/bash.html#index-_005b_005b

- Without this commit
```
[246298 ms] Start: Run in container: /bin/sh -c .devcontainer/boot.sh
Bundle complete! 84 Gemfile dependencies, 242 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
1 installed gem you directly depend on is looking for funding.
  Run `bundle fund` for details
.devcontainer/boot.sh: 3: [[: not found
```

- With this commit : nvm command works as expected
```
[67020 ms] Start: Run in container: /bin/sh -c .devcontainer/boot.sh
Bundle complete! 84 Gemfile dependencies, 242 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
1 installed gem you directly depend on is looking for funding.
  Run `bundle fund` for details
Installing latest LTS version.
Downloading and installing node v22.14.0...
Downloading https://nodejs.org/dist/v22.14.0/node-v22.14.0-linux-arm64.tar.xz...
... snip ...
Computing checksum with sha256sum
Checksums matched!
Now using node v22.14.0 (npm v10.9.2)
yarn install v1.22.22
[1/4] Resolving packages...
[2/4] Fetching packages...
... snip ...
[3/4] Linking dependencies...
warning " > @rails/[email protected]" has unmet peer dependency "trix@^2.0.0".
... snip ...
[4/4] Building fresh packages...
success Saved lockfile.
Done in 11.86s.

What's next:
    Try Docker Debug for seamless, persistent debugging tools in any container or image → docker debug 0ee9dbd1ef72b3d01d1aca37ce62360b8ce0c484d43fe8c6123e7ab63244ea67
    Learn more at https://docs.docker.com/go/debug-cli/
Done. Press any key to close the terminal.
```
Fix rails#54564

Author: yahonda

.devcontainer/boot.sh

@@ -1,6 +1,6 @@
 bundle install
 
-if [[ ! -z "${NVM_DIR}" ]]; then
+if [ -n "${NVM_DIR}" ]; then
   . ${NVM_DIR}/nvm.sh && nvm install --lts
   yarn install
 fi