Add BuildKite build agent

docs/input/docs/reference/build-servers/buildkite.md

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+---
+Order: 40
+Title: Buildkite
+Description: Details on the Buildkite support in GitVersion
+RedirectFrom: docs/build-server-support/build-server/buildkite
+---
+
+If you use [Buildkite][buildkite] then you will have to use GitVersion from the command line as there is currently no GitVersion Buildkite plugin.
+
+## Gotchas
+
+By default Buildkite calls `git fetch` with the flags `-v --prune` which can cause issues on new build agents since branches or tags might not be available locally on the build agent when GitVersion runs. This can be fixed by altering the [Buildkite agent configuration][configuration] either by:
+*   Setting the environment variable `BUILDKITE_GIT_FETCH_FLAGS` to `-v --tags`
+*   Setting configuration value `git-fetch-flags` to `-v --tags` in your agent configuration file
+
+If you are running GitVersion in a docker container make sure to propogate the `BUILDKITE` and `BUILDKITE_BRANCH` environment variables (c.f. example below).
+
+## Example
+
+There are many ways to run GitVersion in a Buildkite pipeline. One way using the GitVersion docker image and using [build meta-data][meta-data] to share version info between build steps. Such a pipeline might look like the following:
+
+```yaml
+env:
+  BUILDKITE_GIT_FETCH_FLAGS: "-v --tags"
+
+steps:
+  - label: "Calculate version"
+    command: buildkite-agent meta-data set "GitVersion_SemVer" $(./dotnet-gitversion -showvariable SemVer)
+    plugins:
+      - docker#v3.9.0:
+        image: "gittools/gitversion"
+        environment:
+          - "BUILDKITE"
+          - "BUILDKITE_BRANCH"
+
+  - wait
+
+  - label: "Use calculated version"
+    command: echo "Calculated version is $(buildkite-agent meta-data get "GitVersion_SemVer")"
+```
+
+Another way could be via the [Buildkite hooks][hooks]. Adding the following line to the `.buildkite/hooks/post-checkout` file:
+
+```shell
+eval $(gitversion | jq -r 'to_entries[] | "buildkite-agent meta-data set GitVersion_\(.key) \(.value)"')
+```
+
+Assuming your Buildkite agent has dotnet and gitversion installed and on the path, all the calculated GitVersion variables will have a corresponding meta-data key set.
+
+[buildkite]: https://buildkite.com/
+[configuration]: https://buildkite.com/docs/agent/v3/hooks
+[hooks]: https://buildkite.com/docs/agent/v3/hooks
+[meta-data]: https://buildkite.com/docs/agent/v3/cli-meta-data

docs/input/docs/reference/build-servers/continua.md

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 ---
-Order: 40
+Order: 50
 Title: Continua CI
 Description: Details on the Continua CI support in GitVersion
 RedirectFrom: docs/build-server-support/build-server/continua

docs/input/docs/reference/build-servers/gitlab.md

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 ---
-Order: 50
+Order: 60
 Title: GitLab CI
 Description: Details on the GitLab CI support in GitVersion
 RedirectFrom: docs/build-server-support/build-server/gitlab

docs/input/docs/reference/build-servers/jenkins.md

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 ---
-Order: 60
+Order: 70
 Title: Jenkins
 Description: Details on the Jenkins support in GitVersion
 RedirectFrom: docs/build-server-support/build-server/jenkins

docs/input/docs/reference/build-servers/myget.md

@@ -1,5 +1,5 @@
 ---
-Order: 70
+Order: 80
 Title: MyGet
 Description: Details on the MyGet support in GitVersion
 RedirectFrom: docs/build-server-support/build-server/myget

docs/input/docs/reference/build-servers/octopus-deploy.md

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 ---
-Order: 80
+Order: 90
 Title: Octopus Deploy
 Description: Details on the Octopus Deploy support in GitVersion
 RedirectFrom: docs/build-server-support/build-server/octopus-deploy

docs/input/docs/reference/build-servers/teamcity.md

@@ -1,5 +1,5 @@
 ---
-Order: 90
+Order: 100
 Title: TeamCity
 Description: Details on the TeamCity support in GitVersion
 RedirectFrom: docs/build-server-support/build-server/teamcity

src/GitVersion.Core.Tests/BuildAgents/BuildKiteTests.cs

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+using GitVersion.BuildAgents;
+using GitVersion.Core.Tests.Helpers;
+using Microsoft.Extensions.DependencyInjection;
+using NUnit.Framework;
+using Shouldly;
+
+namespace GitVersion.Core.Tests.BuildAgents;
+
+[TestFixture]
+public class BuildKiteTests : TestBase
+{
+    private IEnvironment environment;
+    private BuildKite buildServer;
+
+    [SetUp]
+    public void SetUp()
+    {
+        var sp = ConfigureServices(services => services.AddSingleton<BuildKite>());
+        this.environment = sp.GetService<IEnvironment>();
+        this.buildServer = sp.GetService<BuildKite>();
+        this.environment.SetEnvironmentVariable(BuildKite.EnvironmentVariableName, "true");
+    }
+
+    [TearDown]
+    public void TearDown() => this.environment.SetEnvironmentVariable(BuildKite.EnvironmentVariableName, null);
+
+    [Test]
+    public void CanApplyToCurrentContextShouldBeTrueWhenEnvironmentVariableIsSet()
+    {
+        // Act
+        var result = this.buildServer.CanApplyToCurrentContext();
+
+        // Assert
+        result.ShouldBeTrue();
+    }
+
+    [Test]
+    public void CanApplyToCurrentContextShouldBeFalseWhenEnvironmentVariableIsNotSet()
+    {
+        // Arrange
+        this.environment.SetEnvironmentVariable(BuildKite.EnvironmentVariableName, "");
+
+        // Act
+        var result = this.buildServer.CanApplyToCurrentContext();
+
+        // Assert
+        result.ShouldBeFalse();
+    }
+
+    [Test]
+    public void GetCurrentBranchShouldHandleBranches()
+    {
+        // Arrange
+        this.environment.SetEnvironmentVariable("BUILDKITE_BRANCH", MainBranch);
+
+        // Act
+        var result = this.buildServer.GetCurrentBranch(false);
+
+        // Assert
+        result.ShouldBe(MainBranch);
+    }
+
+    [Test]
+    public void GetSetParameterMessageShouldReturnEmptyArray()
+    {
+        // Act
+        var result = this.buildServer.GenerateSetParameterMessage("Foo", "Bar");
+
+        // Assert
+        result.ShouldBeEmpty();
+    }
+
+    [Test]
+    public void GetEmptyGenerateSetVersionMessage()
+    {
+        // Arrange
+        var vars = new TestableVersionVariables("1.0.0");
+
+        // Act
+        var message = this.buildServer.GenerateSetVersionMessage(vars);
+
+        // Assert
+        message.ShouldBeEmpty();
+    }
+}

src/GitVersion.Core/BuildAgents/BuildKite.cs

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+using GitVersion.Logging;
+using GitVersion.OutputVariables;
+
+namespace GitVersion.BuildAgents;
+
+public class BuildKite : BuildAgentBase
+{
+    public BuildKite(IEnvironment environment, ILog log) : base(environment, log)
+    {
+    }
+
+    public const string EnvironmentVariableName = "BUILDKITE";
+
+    protected override string EnvironmentVariable { get; } = EnvironmentVariableName;
+
+    public override bool CanApplyToCurrentContext() => Environment.GetEnvironmentVariable(EnvironmentVariable)?.Equals("true", StringComparison.OrdinalIgnoreCase) ?? false;
+
+    public override string GenerateSetVersionMessage(VersionVariables variables) =>
+        string.Empty; // There is no equivalent function in BuildKite.
+
+    public override string[] GenerateSetParameterMessage(string name, string value) =>
+        Array.Empty<string>(); // There is no equivalent function in BuildKite.
+
+    public override string? GetCurrentBranch(bool usingDynamicRepos) => Environment.GetEnvironmentVariable("BUILDKITE_BRANCH");
+
+    public override bool PreventFetch() => true;
+}