devcontainer/on-create-command: do more stuff.

- fix permissions
- install more gems/formulae
- remove system shellcheck that won't be used

.devcontainer/on-create-command.sh

@@ -1,10 +1,22 @@
 #!/bin/bash
 set -e
 
+# fix permissions so Homebrew and Bundler don't complain
+sudo chmod -R g-w,o-w /home/linuxbrew
+
+# install Homebrew's development gems
+brew install-bundler-gems
+
+# install Homebrew formulae we might need
+brew install shellcheck shfmt gh gnu-tar
+
+# cleanup any mess
+brew cleanup
+
+# install some useful development things
 sudo apt-get update
 sudo apt-get install -y \
   -o Dpkg::Options::=--force-confdef \
   -o Dpkg::Options::=--force-confnew \
   zsh \
-  shellcheck \
   zsh-autosuggestions