switch to debian based powershell image

this matches the version used on our hosts
-preview is used because buster (including the newer nf_tables backend for iptables) isn't available in stretch

Dockerfile

@@ -1,4 +1,4 @@
-FROM mcr.microsoft.com/powershell:latest
+FROM mcr.microsoft.com/powershell:preview-debian-buster-slim
 
 VOLUME /stacks
 
@@ -7,6 +7,7 @@ ENV CHAIN="SWARM-NAT" \
 
 WORKDIR /
 
+# install dependencies
 RUN pwsh -c 'Set-PSRepository -Name "PSGallery" -InstallationPolicy Trusted' \
  && pwsh -c 'Install-Module powershell-yaml' \
  && apt-get update \