update docker image to use 18.04

tools/releaseBuild/Images/microsoft_powershell_ubuntu16.04/Dockerfile

@@ -1,5 +1,22 @@
 # Docker image file that describes an Ubuntu16.04 image with PowerShell and SnapCraft installed from Microsoft APT Repo
-FROM snapcore/snapcraft:stable
+FROM ubuntu:18.04
+
+
+RUN apt-get update && \
+  apt-get dist-upgrade --yes && \
+  apt-get install --yes \
+  curl sudo jq squashfs-tools && \
+  curl -L $(curl -H 'X-Ubuntu-Series: 16' 'https://api.snapcraft.io/api/v1/snaps/details/core' | jq '.download_url' -r) --output core.snap && \
+  mkdir -p /snap/core && unsquashfs -d /snap/core/current core.snap && rm core.snap && \
+  curl -L $(curl -H 'X-Ubuntu-Series: 16' 'https://api.snapcraft.io/api/v1/snaps/details/snapcraft?channel=edge' | jq '.download_url' -r) --output snapcraft.snap && \
+  mkdir -p /snap/snapcraft && unsquashfs -d /snap/snapcraft/current snapcraft.snap && rm snapcraft.snap && \
+  apt remove --yes --purge jq squashfs-tools && \
+  apt-get autoclean --yes && \
+  apt-get clean --yes
+
+COPY bin/snapcraft-wrapper /snap/bin/snapcraft
+
+ENV PATH=/snap/bin:$PATH
 
 # Install apt-utils (for apt-key used later)
 # Install ca-certificates so we can use ssl
@@ -11,13 +28,12 @@ RUN apt-get update && \
     apt-get install -y --no-install-recommends \
         apt-utils \
         ca-certificates \
-        curl \
         apt-transport-https \
         git && \
 # Import the public repository GPG keys for Microsoft
     curl https://packages.microsoft.com/keys/microsoft.asc | apt-key add -  && \
 # Register the Microsoft Ubuntu 16.04 repository with apt
-    curl https://packages.microsoft.com/config/ubuntu/16.04/prod.list | tee /etc/apt/sources.list.d/microsoft.list && \
+    curl https://packages.microsoft.com/config/ubuntu/18.04/prod.list | tee /etc/apt/sources.list.d/microsoft.list && \
 # Update after register new repo
     apt-get update && \
 # Install Powershell
@@ -26,7 +42,7 @@ RUN apt-get update && \
 # Clean up so the layer is small
     apt remove --yes --purge apt-utils apt-transport-https && \
     apt-get autoclean --yes && \
-    apt-get clean --yes && \ 
+    apt-get clean --yes && \
     rm /etc/apt/sources.list.d/microsoft.list
 
 COPY powershell-snap.ps1 /powershell-snap.ps1