Allow Dockerfile from outside build-context

Historically, the Dockerfile had to be insde the build-context, because it was
sent as part of the build-context.

moby/moby@3f6dc81
added support for passing the Dockerfile through stdin, in which case the
contents of the Dockerfile is injected into the build-context.

This patch uses the same mechanism for situations where the location of the
Dockerfile is passed, and its path is outside of the build-context.

Before this change:

    $ mkdir -p myproject/context myproject/dockerfiles && cd myproject
    $ echo "hello" > context/hello
    $ echo -e "FROM busybox\nCOPY /hello /\nRUN cat /hello" > dockerfiles/Dockerfile
    $ docker build --no-cache -f $PWD/dockerfiles/Dockerfile $PWD/context

    unable to prepare context: the Dockerfile (/Users/sebastiaan/projects/test/dockerfile-outside/myproject/dockerfiles/Dockerfile) must be within the build context

After this change:

    $ mkdir -p myproject/context myproject/dockerfiles && cd myproject
    $ echo "hello" > context/hello
    $ echo -e "FROM busybox\nCOPY /hello /\nRUN cat /hello" > dockerfiles/Dockerfile
    $ docker build --no-cache -f $PWD/dockerfiles/Dockerfile $PWD/context

    Sending build context to Docker daemon  2.607kB
    Step 1/3 : FROM busybox
     ---> 6ad733544a63
    Step 2/3 : COPY /hello /
     ---> 9a5ae1c7be9e
    Step 3/3 : RUN cat /hello
     ---> Running in 20dfef2d180f
    hello
    Removing intermediate container 20dfef2d180f
     ---> ce1748f91bb2
    Successfully built ce1748f91bb2

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/image/build.go

@@ -11,6 +11,8 @@ import (
 	"os"
 	"regexp"
 	"runtime"
+	"strings"
+	"path/filepath"
 
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
@@ -206,6 +208,14 @@ func runBuild(dockerCli command.Cli, options buildOptions) error {
 		buildCtx, relDockerfile, err = build.GetContextFromReader(dockerCli.In(), options.dockerfileName)
 	case isLocalDir(specifiedContext):
 		contextDir, relDockerfile, err = build.GetContextFromLocalDir(specifiedContext, options.dockerfileName)
+		if err == nil && strings.HasPrefix(relDockerfile, ".."+string(filepath.Separator)) {
+			// Dockerfile is outside of build-context; read the Dockerfile and pass it as dockerfileCtx
+			dockerfileCtx, err = os.Open(relDockerfile)
+			if err != nil {
+				return errors.Errorf("unable to open Dockerfile: %v", err)
+			}
+			defer dockerfileCtx.Close()
+		}
 	case urlutil.IsGitURL(specifiedContext):
 		tempDir, relDockerfile, err = build.GetContextFromGitURL(specifiedContext, options.dockerfileName)
 	case urlutil.IsURL(specifiedContext):
@@ -253,15 +263,15 @@ func runBuild(dockerCli command.Cli, options buildOptions) error {
 		}
 	}
 
-	// replace Dockerfile if it was added from stdin and there is archive context
+	// replace Dockerfile if it was added from stdin or a file outside the build-context, and there is archive context
 	if dockerfileCtx != nil && buildCtx != nil {
 		buildCtx, relDockerfile, err = build.AddDockerfileToBuildContext(dockerfileCtx, buildCtx)
 		if err != nil {
 			return err
 		}
 	}
 
-	// if streaming and dockerfile was not from stdin then read from file
+	// if streaming and Dockerfile was not from stdin then read from file
 	// to the same reader that is usually stdin
 	if options.stream && dockerfileCtx == nil {
 		dockerfileCtx, err = os.Open(relDockerfile)

cli/command/image/build/context.go

@@ -167,6 +167,10 @@ func GetContextFromGitURL(gitURL, dockerfileName string) (string, string, error)
 		return "", "", err
 	}
 	relDockerfile, err := getDockerfileRelPath(absContextDir, dockerfileName)
+	if err == nil && strings.HasPrefix(relDockerfile, ".."+string(filepath.Separator)) {
+		return "", "", errors.Errorf("the Dockerfile (%s) must be within the build context", dockerfileName)
+	}
+
 	return absContextDir, relDockerfile, err
 }
 
@@ -318,10 +322,6 @@ func getDockerfileRelPath(absContextDir, givenDockerfile string) (string, error)
 		return "", errors.Errorf("unable to get relative Dockerfile path: %v", err)
 	}
 
-	if strings.HasPrefix(relDockerfile, ".."+string(filepath.Separator)) {
-		return "", errors.Errorf("the Dockerfile (%s) must be within the build context", givenDockerfile)
-	}
-
 	return relDockerfile, nil
 }