The $0 value is used in many CLI libraries to determine the name of the
application, when displaying help and error messages.
Without setting this value, it defaults to `gem` which can be confusing.
Before:
```
$ gem exec kamal help
Commands:
gem accessory # Manage accessories (db/redis/search)
gem app # Manage application
gem audit # Show audit log from servers
gem build # Build application image
gem config # Show combined config (including secrets!)
gem deploy # Deploy app to servers
gem details # Show details about all containers
gem docs [SECTION] # Show Kamal configuration documentation
gem help [COMMAND] # Describe available commands or one specific command
gem init # Create config stub in config/deploy.yml and secrets stub in .kamal
gem lock # Manage the deploy lock
gem proxy # Manage kamal-proxy
gem prune # Prune old application images and containers
gem redeploy # Deploy app to servers without bootstrapping servers, starting kamal-proxy, pruning, and registry login
gem registry # Login and -out of the image registry
gem remove # Remove kamal-proxy, app, accessories, and registry session from servers
gem rollback [VERSION] # Rollback app to VERSION
gem secrets # Helpers for extracting secrets
gem server # Bootstrap servers with curl and Docker
gem setup # Setup all accessories, push the env, and deploy app to servers
gem upgrade # Upgrade from Kamal 1.x to 2.0
gem version # Show Kamal version
```
After:
```
$ gem exec kamal help
Commands:
kamal accessory # Manage accessories (db/redis/search)
kamal app # Manage application
kamal audit # Show audit log from servers
kamal build # Build application image
kamal config # Show combined config (including secrets!)
kamal deploy # Deploy app to servers
kamal details # Show details about all containers
kamal docs [SECTION] # Show Kamal configuration documentation
kamal help [COMMAND] # Describe available commands or one specific command
kamal init # Create config stub in config/deploy.yml and secrets stub in .kamal
kamal lock # Manage the deploy lock
kamal proxy # Manage kamal-proxy
kamal prune # Prune old application images and containers
kamal redeploy # Deploy app to servers without bootstrapping servers, starting kamal-proxy, pruning, and registry login
kamal registry # Login and -out of the image registry
kamal remove # Remove kamal-proxy, app, accessories, and registry session from servers
kamal rollback [VERSION] # Rollback app to VERSION
kamal secrets # Helpers for extracting secrets
kamal server # Bootstrap servers with curl and Docker
kamal setup # Setup all accessories, push the env, and deploy app to servers
kamal upgrade # Upgrade from Kamal 1.x to 2.0
kamal version # Show Kamal version
```
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