Suggest variable names as completion items

[rcjsuen]

If the user types in a $ or invokes completion after a $, we should suggest a list of variables to the client. Since the brace syntax gets around whitespace issues, the brace syntax is what will be suggested. However, should a user start typing after the $ directly such as $v, then $var should be suggested even if ${var} was what was originally suggested.

By default, Docker includes the following variables that can be used without first declaring them in an ARG instruction:

• HTTP_PROXY
• http_proxy
• HTTPS_PROXY
• https_proxy
• FTP_PROXY
• ftp_proxy
• NO_PROXY
• no_proxy

[rcjsuen]

However, should a user start typing after the $ directly such as $v, then $var should be suggested even if ${var} was what was originally suggested.

Doesn't look like this is exactly possible. This is because clients may opt to filter the initial list after the user has started typing instead of going to the trouble of asking the server for items again. This seems fair from a performance perspective.