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

@@ -9,8 +9,10 @@ import (
  "io"
  "io/ioutil"
  "os"
+ "path/filepath"
  "regexp"
  "runtime"
+ "strings"
 
  "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
 }