[SPARK-26687][K8S] Fix handling of custom Dockerfile paths

Fixes a bug with bin/docker-image-tool.sh not resolving relative paths
to custom Dockerfiles which results in docker builds failing with
unhelpful errors because the builds are run from inside the temporary
build context directory.  Therefore valid relative paths where the
script are invoked becomes invalid when the actual docker build is
attempted.

Author: rvesse

bin/docker-image-tool.sh

@@ -67,6 +67,16 @@ function docker_push {
   fi
 }
 
+function resolve_file {
+  local FILE=$1
+  if [ -n "$FILE" ]; then
+    local DIR=$(dirname $FILE)
+    DIR=$(cd $DIR && pwd)
+    FILE="${DIR}/$(basename $FILE)"
+  fi
+  echo $FILE
+}
+
 # Create a smaller build context for docker in dev builds to make the build faster. Docker
 # uploads all of the current directory to the daemon, and it can get pretty big with dev
 # builds that contain test log files and other artifacts.
@@ -257,9 +267,9 @@ while getopts f:p:R:mr:t:nb:u: option
 do
  case "${option}"
  in
- f) BASEDOCKERFILE=${OPTARG};;
- p) PYDOCKERFILE=${OPTARG};;
- R) RDOCKERFILE=${OPTARG};;
+ f) BASEDOCKERFILE=$(resolve_file ${OPTARG});;
+ p) PYDOCKERFILE=$(resolve_file ${OPTARG});;
+ R) RDOCKERFILE=$(resolve_file ${OPTARG});;
  r) REPO=${OPTARG};;
  t) TAG=${OPTARG};;
  n) NOCACHEARG="--no-cache";;