Have .dockerignore support Dockerfile/.dockerignore

If .dockerignore mentions either then the client will send them to the
daemon but the daemon will erase them after the Dockerfile has been parsed
to simulate them never being sent in the first place.

an events test kept failing for me so I tried to fix that too

Closes #8330

Signed-off-by: Doug Davis <dug@us.ibm.com>

tarsum/builder_context.go

@@ -0,0 +1,20 @@
+package tarsum
+
+// This interface extends TarSum by adding the Remove method.  In general
+// there was concern about adding this method to TarSum itself so instead
+// it is being added just to "BuilderContext" which will then only be used
+// during the .dockerignore file processing - see builder/evaluator.go
+type BuilderContext interface {
+	TarSum
+	Remove(string)
+}
+
+func (bc *tarSum) Remove(filename string) {
+	for i, fis := range bc.sums {
+		if fis.Name() == filename {
+			bc.sums = append(bc.sums[:i], bc.sums[i+1:]...)
+			// Note, we don't just return because there could be
+			// more than one with this name
+		}
+	}
+}