MSBuild error when creating package using Linux docker image

Environment:

Ubuntu18.04

OpenTAP 9.7.0

Docker image opentapio/opentap:9.7-ubuntu18.04

Steps to reproduce:

1. Create/find a project that uses the opentap nuget package to create a tappackage
2. run `dotnet publish -c Release

Actual result: Excerpt from a gitlab CI build:

$ dotnet publish SshSteps.sln -c Release
Microsoft (R) Build Engine version 16.2.37902+b5aaefc9f for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.
  Restore completed in 3.02 sec for /builds/OpenTAP/Plugins/keysight/sshsteps/OpenTap.Plugins.Ssh.csproj.
  OpenTap.Plugins.Ssh -> /builds/OpenTAP/Plugins/keysight/sshsteps/bin/Release/OpenTap.Plugins.Ssh.dll
/root/.nuget/packages/opentap/9.7.0/build/OpenTap.targets(53,5): error : /bin/bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8) [/builds/OpenTAP/Plugins/keysight/sshsteps/OpenTap.Plugins.Ssh.csproj]
...
  Creating OpenTAP package.
  OpenTAP plugin package 'Ssh.0.3.0-alpha.1.3+f1ab2f2b.sshResources.TapPackage' containing 'Ssh' successfully created.
  Installed 'Ssh' (/builds/OpenTAP/Plugins/keysight/sshsteps/bin/Release/Packages/Ssh/package.xml)
/root/.nuget/packages/opentap/9.7.0/build/OpenTap.targets(53,5): error MSB3073: The command "./tap package create "/builds/OpenTAP/Plugins/keysight/sshsteps//package.xml" --install" exited with code -1. [/builds/OpenTAP/Plugins/keysight/sshsteps/OpenTap.Plugins.Ssh.csproj]
Running after_script
00:01
Uploading artifacts for failed job
00:02
ERROR: Job failed: exit code 1

Full build log here: https://gitlab.com/OpenTAP/Plugins/keysight/sshsteps/-/jobs/529075216

Expected result:

No warnings about locales, and 0 error code.