Add buildContext property to Platform manifest model

src/ImageBuilder.Models/Manifest/Platform.cs

@@ -28,6 +28,11 @@ public class Platform
         "to override variables defined in the Dockerfile.")]
     public IDictionary<string, string> BuildArgs { get; set; } = new Dictionary<string, string>();
 
+    [Description(
+        "Relative path to the build context directory for the `docker build` command. " +
+        "If not specified, the directory containing the Dockerfile is used.")]
+    public string? BuildContext { get; set; }
+
     [Description(
         "Relative path to the associated Dockerfile. This can be a file or a " +
         "directory. If it is a directory, the file name defaults to Dockerfile."

src/ImageBuilder.Tests/Models/PlatformSerializationTests.cs

@@ -50,6 +50,7 @@ public void FullyPopulatedPlatform_Bidirectional()
                 ["DOTNET_VERSION"] = "8.0.0",
                 ["ASPNET_VERSION"] = "8.0.0"
             },
+            BuildContext = "src/runtime/8.0/jammy",
             Dockerfile = "src/runtime/8.0/jammy/arm64v8/Dockerfile",
             DockerfileTemplate = "src/runtime/Dockerfile.linux.template",
             OS = OS.Linux,
@@ -71,6 +72,7 @@ public void FullyPopulatedPlatform_Bidirectional()
                 "DOTNET_VERSION": "8.0.0",
                 "ASPNET_VERSION": "8.0.0"
               },
+              "buildContext": "src/runtime/8.0/jammy",
               "dockerfile": "src/runtime/8.0/jammy/arm64v8/Dockerfile",
               "dockerfileTemplate": "src/runtime/Dockerfile.linux.template",
               "os": "Linux",
@@ -95,6 +97,7 @@ public void FullyPopulatedPlatform_RoundTrip()
         {
             Architecture = Architecture.ARM,
             BuildArgs = new Dictionary<string, string> { ["VERSION"] = "8.0" },
+            BuildContext = "src",
             Dockerfile = "src/Dockerfile",
             DockerfileTemplate = "src/Dockerfile.template",
             OS = OS.Linux,
@@ -266,6 +269,7 @@ private static void AssertPlatformsEqual(Platform expected, Platform actual)
     {
         Assert.Equal(expected.Architecture, actual.Architecture);
         Assert.Equal(expected.BuildArgs, actual.BuildArgs);
+        Assert.Equal(expected.BuildContext, actual.BuildContext);
         Assert.Equal(expected.Dockerfile, actual.Dockerfile);
         Assert.Equal(expected.DockerfileTemplate, actual.DockerfileTemplate);
         Assert.Equal(expected.OS, actual.OS);

src/ImageBuilder.Tests/PlatformInfoTests.cs

@@ -83,5 +83,44 @@ private void ValidateGetOSDisplayName(OS os, string osVersion, string expectedDi
 
             Assert.Equal(expectedDisplayName, platformInfo.GetOSDisplayName());
         }
+
+        [Fact]
+        public void BuildContextPath_DefaultsToDockerfileDirectory()
+        {
+            using TempFolderContext tempFolderContext = TestHelper.UseTempFolder();
+
+            string dockerfilePath = DockerfileHelper.CreateDockerfile("src/runtime/os", tempFolderContext);
+
+            Platform platform = CreatePlatform(dockerfilePath, [ "test" ]);
+
+            VariableHelper variableHelper = new(new Manifest(), Mock.Of<IManifestOptionsInfo>(), null);
+            PlatformInfo platformInfo = PlatformInfo.Create(
+                platform, "", "test", variableHelper, tempFolderContext.Path);
+
+            string expectedContextPath = PathHelper.NormalizePath(
+                Path.Combine(tempFolderContext.Path, "src/runtime/os"));
+
+            Assert.Equal(expectedContextPath, platformInfo.BuildContextPath);
+        }
+
+        [Fact]
+        public void BuildContextPath_UsesExplicitBuildContext()
+        {
+            using TempFolderContext tempFolderContext = TestHelper.UseTempFolder();
+
+            string dockerfilePath = DockerfileHelper.CreateDockerfile("src/runtime/os", tempFolderContext);
+
+            Platform platform = CreatePlatform(dockerfilePath, [ "test" ]);
+            platform.BuildContext = "src";
+
+            VariableHelper variableHelper = new(new Manifest(), Mock.Of<IManifestOptionsInfo>(), null);
+            PlatformInfo platformInfo = PlatformInfo.Create(
+                platform, "", "test", variableHelper, tempFolderContext.Path);
+
+            string expectedContextPath = PathHelper.NormalizePath(
+                Path.Combine(tempFolderContext.Path, "src"));
+
+            Assert.Equal(expectedContextPath, platformInfo.BuildContextPath);
+        }
     }
 }

src/ImageBuilder/ViewModel/ModelExtensions.cs

@@ -138,6 +138,11 @@ private static void ValidatePlatform(Platform platform, string manifestDirectory
         {
             ValidateFileReference(platform.ResolveDockerfilePath(manifestDirectory), manifestDirectory);
             ValidateFileReference(platform.DockerfileTemplate, manifestDirectory);
+
+            if (platform.BuildContext is not null)
+            {
+                ValidatePathIsRelative(platform.BuildContext);
+            }
         }
 
         private static void ValidateUniqueTags(Repo repo)

src/ImageBuilder/ViewModel/PlatformInfo.cs

@@ -59,7 +59,9 @@ private PlatformInfo(Platform model, string baseOsVersion, string fullRepoModelN
 
             string dockerfileWithBaseDir = Path.Combine(baseDirectory, model.ResolveDockerfilePath(baseDirectory));
             DockerfilePath = PathHelper.NormalizePath(dockerfileWithBaseDir);
-            BuildContextPath = PathHelper.NormalizePath(Path.GetDirectoryName(dockerfileWithBaseDir));
+            BuildContextPath = model.BuildContext is not null
+                ? PathHelper.NormalizePath(Path.Combine(baseDirectory, model.BuildContext))
+                : PathHelper.NormalizePath(Path.GetDirectoryName(dockerfileWithBaseDir));
             DockerfilePathRelativeToManifest = PathHelper.TrimPath(baseDirectory, DockerfilePath);
 
             if (model.DockerfileTemplate != null)