Dockerfile

FROM microsoft/aspnet
WORKDIR /inetpub/wwwroot
COPY . .
ENTRYPOINT ["powershell.exe", ".\\Startup.ps1"]

Set-WebConfigSettings.ps1

param (
    [string]$webConfig = ".\Web.config.test"
)

$doc = (Get-Content $webConfig) -as [Xml];

$appSettingPrefix = "APPSETTING_";
$connectionStringPrefix = "CONNSTR_";

Get-ChildItem env:* | ForEach-Object {
    if ($_.Key.StartsWith($appSettingPrefix)) {
        $key = $_.Key.Substring($appSettingPrefix.Length);
        $appSetting = $doc.configuration.appSettings.add | Where-Object {$_.key -eq $key};
        if ($appSetting) {
            $appSetting.value = $_.Value;
            Write-Host "Replaced appSetting" $_.Key $_.Value;
        }
    }
    if ($_.Key.StartsWith($connectionStringPrefix)) {
        $key = $_.Key.Substring($connectionStringPrefix.Length);
        $connStr = $doc.configuration.connectionStrings.add | Where-Object {$_.name -eq $key};
        if ($connStr) {
            $connStr.connectionString = $_.Value;
            Write-Host "Replaced connectionString" $_.Key $_.Value;
        }
    }
}

$doc.Save($webConfig);

Startup.ps1

.\Set-WebConfigSettings.ps1 -webConfig .\Web.config
C:\ServiceMonitor.exe w3svc