Variables not replaced in attach config

[chrmarti]

[connor4312]

@chrmarti can you add verification steps?

[chrmarti]

To verify:

• docker run -d ubuntu sleep infinity
• Attach to the new container from the Remote Explorer.
• Once attached: Run command Remote-Containers: Open Container Configuration File.
• Change the file to:

{
	"remoteEnv": {
		"TEST1": "${containerEnv:PATH}",
		"TEST2": "${localEnv:PATH}"
	}
}

• Reload the window.
• Open integrated terminal.
• Check set | egrep '^TEST' shows the two environment variables with the embedded variables correctly replaced.