You might find #45 kind of familiar.

Can't explain what is going on here. Will investigate further. Apologies for the noise.
@lextm - yes, I found your testing in #45 useful.

I've done some more testing, but failed to get the CI workflow to run without error. The current main one has 4 errors, all apparently from changes in the fsharp versions in the github ubuntu 20.04 environment. Installing software explicitly, which seemed to work initially, now fails with over 100 errors, some such as
DOWNLOADNUPKG : Ssl error : 1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED which I have no idea how to tackle!
@lextm: I've copied your amended workflow from #45 to try out using a container, which would hopefully start from a more basic ubuntu repo, but even this now fails to run.
Using VMs, ubuntu 20.04, ubuntu 21.04, and fedora 33 all build and run with no problems, following the Wiki instructions, so it appears to be the github CI workflow environment which is at the root of the problem.

The cause is rather simple that Microsoft is retiring NuGet.org v2 API, https://docs.microsoft.com/en-us/nuget/nuget-org/overview-nuget-org#api-endpoint-for-nugetorg

If you override NuGet settings to use v3, I think it should work again.

I've updated the CI workflow file to use the docker image "ubuntu:20.04" and explicitly installed all the necessary packages into that. After some trial-and-error testing in my CI_tests branch, I have a working version that passes the CI runs.

However, after transferring this version to the fix_CI_env branch, the same code fails, with multiple errors of the form DOWNLOADNUPKG : Ssl error : 1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED as seen in the runs above. I'm not sure what to make of this: usually intermittent failures suggest concurrency issues, but perhaps there's another possibility?

@lextm: following your suggestion, I checked the dotdevelop/.NuGet.config file, but it appears to be using v3 already. Is there somewhere else to look?

The numerous Ssl error : 1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED errors seem to have reduced after installing ca-certificates-mono, but some CI runs still fail, while others succeed, for no obvious reason.
The logs show errors such as "- Something went wrong downloading StrongNamer 0.0.8" and " - Something went wrong downloading FSharp.Compiler.Service 31.0", which may be due to the change from FSharp 4.5 to FSharp 5.0 (or perhaps not!)
Any suggestions would be appreciated.

@hwthomas ready to merge, what do you say?

@lytico - I've had several consistent runs now in my fix_CI_env and CI_tests branches, so I'm happy if you are.

The Ssl error : 1000007d problems seemed to be caused by trying to access nuget.org/v2 sites, but I've not been able to track down where these calls originate in the build, and I still don't understand how or why the failures were intermittent.
I've squashed all the 'trial-and-error' commits into a single one to make life a bit simpler

how or why the failures were intermittent.

i have this on other CI's as well, seems to be a workload problem on github or nuget.org, maybe ...

I checked the dotdevelop/.NuGet.config file, but it appears to be using v3 already

but maybe on the tons of subprojects they are not?

i have this on other CI's as well, seems to be a workload problem on github or nuget.org, maybe ...

That's interesting. I was assuming it was a concurrency issue, but couldn't find a way to run the CI in a single thread to check. At least I'm not the only one to have had a problem!

but maybe on the tons of subprojects they are not?

A full search of the repo shows that nuget.org/api/v2 only occurs in the external/fsharpbinding project, in files paket.dependencies and .nuget/NuGet.targets
Successful run CI#80 (https://github.com/dotdevelop/dotdevelop/runs/3232143181?check_suite_focus=true#step:6:4311) shows all the fsharpbinding dependencies being restored without problem, and without needing to download from https://www.nuget.org/api/v2/.
In contrast, failing run CI#78 (https://github.com/dotdevelop/dotdevelop/runs/3161054112?check_suite_focus=true#step:4:4324) tries to access these sources and fails.

@lytico As this is a specific fsharpbinding problem, would it be better to deal with it in a separate Issue/PR?

For this PR though, basing the CI workflow on a minimal docker image still seems worthwhile, as it gives finer control over the software environment, particularly if/when mono and msbuild versions change as .NET5 stabilises and progresses. It also links the CI to the 'Setting up a Build Environment' Wiki pages more clearly.

A full search of the repo shows that nuget.org/api/v2 only occurs in the external/fsharpbinding project, in files paket.dependencies and .nuget/NuGet.targets

what if changing to nuget.org/api/v3 in this files?
should be another PR

what if changing to nuget.org/api/v3 in this files?

Using "https://api.nuget.org/v3/index.json" as the url in the paket.dependencies file gave an error:-

Starting full restore process.
Performance:
- Runtime: 597 milliseconds
Paket failed with
-> The NuGet source https://www.nuget.org/api/v2 for package FSharp.Compiler.Service was not found in the paket.dependencies file with sources [NuGetV3 {Url = "https://api.nuget.org/v3/index.json"; ...}

Where could the reference https://www.nuget.org/api/v2 for package FSharp.Compiler.Service be coming from? As far as I can see it does not occur anywhere in the repo, so how to get rid of it?

Any suggestions or pointers for further investigation/understanding would be much appreciated. Until then I'll leave the PR.