Add Codespaces for source build investigations (#15285)

.devcontainer/vmr-source-build/README.md

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+<!--
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+##    ##  ##       ##     ## ##     ##       ##    ##     ##  ##  ##    ##
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+-->
+
+This Codespace can help you debug the source build of .NET. In case you have run this from a
+`dotnet/installer` PR branch, it will contain the VMR (`dotnet/dotnet`) checked out into
+`/workspaces/dotnet` with the PR changes pulled into it. You can then attempt to source-build
+the VMR which is what the VMR leg in the installer PR build doing. This build takes about 45
+minutes and, after completion, produces an archived .NET SDK in `/workspaces/artifacts/x64/Release`.
+
+To build the VMR, run following:
+```bash
+cd /workspaces/dotnet
+unset RepositoryName
+./build.sh --online
+```
+
+> Please note that, at this time, the build modifies some of the checked-in sources so it might
+be preferential to rebuild the Codespace between attempts.
+
+For more details, see the instructions at https://github.com/dotnet/dotnet.

.devcontainer/vmr-source-build/devcontainer.json

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+// Container suitable for investigating issues with source build
+// Contains the VMR (dotnet/dotnet) with applied changes from the current PR
+// The container supports source-building the SDK
+{
+    "name": "VMR with PR changes",
+    "image": "mcr.microsoft.com/dotnet-buildtools/prereqs:fedora-36",
+    "hostRequirements": {
+        // A completely source built .NET is >64 GB with all the repos/artifacts
+        "storage": "128gb"
+    },
+    "customizations": {
+        "vscode": {
+            "extensions": [
+                "ms-dotnettools.csharp"
+            ]
+        },
+        "codespaces": {
+            "openFiles": [
+                ".devcontainer/vmr-source-build/README.md"
+            ]
+        }
+    },
+    "onCreateCommand": "${containerWorkspaceFolder}/.devcontainer/vmr-source-build/init.sh"
+}

.devcontainer/vmr-source-build/init.sh

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+#!/usr/bin/env bash
+
+set -ex
+
+source="${BASH_SOURCE[0]}"
+script_root="$( cd -P "$( dirname "$source" )" && pwd )"
+
+# TODO: Remove once we have a new version of the image
+dnf -y install openssl1.1
+
+installer_dir=$(realpath "$script_root/../..")
+workspace_dir=$(realpath "$installer_dir/../")
+tmp_dir=$(realpath "$workspace_dir/tmp")
+vmr_dir=$(realpath "$workspace_dir/dotnet")
+
+mkdir -p "$tmp_dir"
+
+# Codespaces performs a shallow fetch only
+git -C "$installer_dir" fetch --all --unshallow
+
+# We will try to figure out, which branch is the current (PR) branch based off of
+# We need this to figure out, which VMR branch to use
+vmr_branch=$(git log --pretty=format:'%D' HEAD^ | grep 'origin/' | head -n1 | sed 's@origin/@@' | sed 's@,.*@@')
+
+"$installer_dir/eng/vmr-sync.sh" \
+    --vmr "$vmr_dir"             \
+    --tmp "$tmp_dir"             \
+    --branch "$vmr_branch"       \
+    --debug
+
+# Run prep.sh
+unset RepositoryName
+pushd "$vmr_dir"
+./prep.sh
+popd