If I explicitly configure the Container-Instance's DNS servers .... it works. And fortunately Bicep can figure out those DNS-servers of my Virtual Network for me.
resource resourceVirtualNetwork 'Microsoft.Network/virtualNetworks@2021-02-01' existing = {
name: virtualNetwork_Name
scope: resourceGroup(virtualNetwork_ResourceGroup_Name)
}resource containerGroup 'Microsoft.ContainerInstance/containerGroups@2021-09-01' = {
name: containergroup_Name
location: location
properties: {
dnsConfig:{
nameServers: resourceVirtualNetwork.properties.dhcpOptions.dnsServers
}
containers: [
{
name: containergroup_Name
properties: {
image: image
........
.........