add Dockerfile and shebang

fixes #7

Dockerfile

@@ -0,0 +1,7 @@
+FROM mcr.microsoft.com/powershell:alpine-3.8
+
+USER nobody
+
+COPY bot.ps1 /
+
+ENTRYPOINT /bot.ps1

bot.ps1

@@ -1,3 +1,5 @@
+#!/usr/bin/pwsh
+
 #alternatively read params from $env
 #for more flexible Dockerfile support
 param (