read parameters optionally from $env and $env+_FILE

fixes #6

bot.ps1

@@ -1,15 +1,18 @@
+#alternatively read params from $env
+#for more flexible Dockerifle support
 param (
-    [Parameter(Mandatory=$true)]
+    [ValidateNotNullOrEmpty()]
     [uri]
-    $HomeServer,
+    $HomeServer = $env:HOMESERVER,
 
-    [Parameter(Mandatory=$true)]
+    [ValidateNotNullOrEmpty()]
     [string]
-    $UserId,
+    $UserId = $env:USERID,
 
-    [Parameter(Mandatory=$true)]
+    [ValidateNotNullOrEmpty()]
     [securestring]
-    $AccessToken
+    $AccessToken = ((Get-Content -Path $env:ACCESSTOKEN_FILE -Raw).Trim() `
+                    | ConvertTo-SecureString -AsPlainText -Force)
 )
 $ErrorActionPreference = 'Stop'