This is the container with the extra volume that won't restore databases. The SQL Notebook includes the error message I'm receiving.

TestContainer64.ipynb

TestContainer64.ps1

<#####################################################################################>
mkdir C:\SQLData\Docker\SQLDev64
<##>
#Requires -Modules SqlServer
<# 0A) Before any of this can work, you must have Docker Destop running. 
    You must also have the latest SqlServer module installed from the PowerShell Gallery.
Install-Module SqlServer
#>
Import-Module SqlServer

<# 0B) Use this code to download the AdventureWorks2016.bak file from GitHub: #>
$BakURL = "https://github.com/Microsoft/sql-server-samples/releases/download/adventureworks/AdventureWorks2016.bak";
$BakFile = "$($Home)/Downloads/AdventureWorks2016.bak";
Invoke-WebRequest -Uri $BakURL -OutFile $BakFile;

<# 1) Create a SQL-on-Linux Docker Container with just the code below.
    NOTE: You may want to change the password used for the SA account.
    You may also want to change the Volumne path: "C:\SQLData\Docker\SQLDev64"  #>
docker run -d -p 10064:1433 -v C:\SQLData\Docker\SQLDev64:/sqlserver -v C:\SQLData\Docker\SQLDev64:/var/opt/mssql/data/ -e ACCEPT_EULA=Y -e SA_PASSWORD=Test1234 --name testcontainer64 mcr.microsoft.com/mssql/server:2019-CTP3.0-ubuntu

<# 2) Copy the backup file to the directory your container volume is mapped to.
    Make sure you use the location you stored the .bak file in. #>
Copy-Item -Path "$($Home)\Downloads\AdventureWorks2016.bak" -Destination C:\SQLData\Docker\SQLDev64

<# Check to make sure you can successfully connect to your new SQL instance runnin in a Container.
    Note: you should only see the four system databases at this point. #>
$Sql64Cred = (Get-Credential sa)
#Get-SqlDatabase -ServerInstance 'localhost,10064' -Credential $Sql64Cred
#docker exec -it testcontainer64 bash

Start-Sleep -Seconds 4

<# Use the Get-SqlDatabase cmdlet to see all the databases on the instance. #>
Get-SqlDatabase -ServerInstance 'localhost,10064' -Credential $Sql64Cred

<# 3) Restore the AdventureWorks2016 database #>
Restore-SqlDatabase -ServerInstance 'localhost,10064' -Credential $Sql64Cred -BackupFile '/sqlserver/AdventureWorks2016.bak' -Database 'AdventureWorks2016' -AutoRelocateFile

<# Use the Get-SqlDatabase cmdlet to see all the databases on the instance. #>
Get-SqlDatabase -ServerInstance 'localhost,10064' -Credential $Sql64Cred

<# 4) This portion allows you to grab all four AdventureWorksDW sample databases from GitHub, 
#    then downloads and the .bak file.
#    After the .bak files are downloaded you restore the instance specified. #>
#$releases = Invoke-RestMethod https://api.github.com/repos/microsoft/sql-server-samples/releases
#$BaksToDownload = ($releases | where { $_.name -eq 'AdventureWorks sample databases' -or $_.name -eq 'Wide World Importers sample database v1.0' }).assets | 
#WHERE { $_.name -like 'AdventureWorksDW201*bak' -and $_.name -notlike '*EXT*' } |
#SELECT name, browser_download_url, size, updated_at#

#FOREACH ( $BakInfo in $BaksToDownload )
#{
#"$($BakInfo.name)";
#Invoke-WebRequest -Uri $BakInfo.browser_download_url -OutFile "C:\SQLData\Docker\SQLDev64\$($BakInfo.name)"
#Restore-SqlDatabase -ServerInstance 'localhost,10064' -Credential $Sql64Cred -BackupFile "/sqlserver/$($BakInfo.name)" -Database ($BakInfo.name -replace '.bak') -AutoRelocateFile
#};#

#Get-SqlDatabase -ServerInstance 'localhost,10064' -Credential $Sql64Cred
#<# Clean-up:
#docker stop testcontainer64
#docker rm testcontainer64
#dir -Path C:\SQLData\Docker\SQLDev64 -Filter AdventureWorks*.bak | Remove-Item
#>