Improved build.sh Docker build script

 - Changed most variables from hard-coded export's, so that they only use their default value if
   the variable wasn't already set in the environment

 - Refactored most lines to follow shellscript standards, e.g. using `${}` where appropriate, quoting
   arguments and variables, etc.

 - Added some additional variables to make it easier to adjust individual portions of the docker tag
   through environment variables

 - Added basic information comment block to the top of the file explaining how to use the build script,
   and how to adjust the docker tag

 - Added a bash shebang and made the file executable

 - Added support for overriding the docker tag by passing an argument

 - Added `$DIR` variable, allowing `build.sh` to be ran from any working directory, without running
   into path context problems.

build.sh

-echo Build started on `date`
-export IMAGE_TAG=`git ls-remote --heads origin | grep $(git rev-parse HEAD) | cut -d / -f 3`
+#!/usr/bin/env bash
+##############################################################################################
+# Quick Docker build script for Jussi. Builds Jussi as a docker container.
+#
+# By default, the image will be tagged as 'hive/jussi:TAG' - where 'TAG' is the current
+# Git branch / tag. 
+# If 'master' is checked out, then TAG will be changed to 'latest'.
+#
+# You can override the Docker image tag like so:
+# 
+#   # These will both simply tag the image as 'jussi' - no user/org, and no version tag
+#   # (the version tag would default to :latest)
+#   ./build.sh jussi
+#   DK_TAG_FULL=jussi ./build.sh
+#
+#   # This will tag the image as 'someguy123/jussi:v1.2.3'
+#   IMAGE_USER="someguy123" IMAGE_TAG="v1.2.3" ./build.sh
+#
+#   # This will tag the image as 'example/my-jussi:TAG' - where TAG will be 
+#   # automatically determined from the git branch/tag.
+#   REPO_NAME="my-jussi" IMAGE_USER="example" ./build.sh
+#
+#   # You can override the entire USER/REPO portion of the tag by setting IMAGE_REPO_NAME
+#   # This would tag the image as 'jollypirate/custom-jussi:TAG' (TAG from git branch/tag)
+#   IMAGE_REPO_NAME="jollypirate/custom-jussi" ./build.sh
+#
+##############################################################################################
+
+# This variable detects the folder containing this script, allowing us to reference the containing folder,
+# making it possible to run build.sh from any other folder, without causing path issues.
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+
+echo "Build started on $(date)"
+
+# Enter the folder containing this script
+cd "$DIR"
+
+: ${IMAGE_USER="hive"}
+
+[ -z ${IMAGE_TAG+x} ] && IMAGE_TAG=`git ls-remote --heads origin | grep $(git rev-parse HEAD) | cut -d / -f 3`
 if [ "$IMAGE_TAG" = "master" ] ; then export IMAGE_TAG=latest ; fi
-export REPO_PATH=`git rev-parse --show-toplevel`
-export REPO_NAME=`basename $REPO_PATH`
-export IMAGE_REPO_NAME="steemit/$REPO_NAME"
-export SOURCE_COMMIT=`git rev-parse HEAD`
-echo Building branch $IMAGE_TAG from $IMAGE_REPO_NAME
-docker build . -t $IMAGE_REPO_NAME:$IMAGE_TAG --build-arg SOURCE_COMMIT="${SOURCE_COMMIT}" --build-arg DOCKER_TAG="${IMAGE_TAG}"
+[ -z ${REPO_PATH+x} ] && REPO_PATH=`git rev-parse --show-toplevel`
+[ -z ${REPO_NAME+x} ] && REPO_NAME=`basename $REPO_PATH`
+: ${IMAGE_REPO_NAME="${IMAGE_USER}/${REPO_NAME}"}
+SOURCE_COMMIT=`git rev-parse HEAD`
+
+
+if (( $# > 0 )); then
+    echo -e "\nUsing docker tag specified as first argument: $1 \n"
+    DK_TAG_FULL="$1"
+fi
+
+: ${DK_TAG_FULL="${IMAGE_REPO_NAME}:${IMAGE_TAG}"}
+
+export IMAGE_USER IMAGE_TAG REPO_PATH REPO_NAME IMAGE_REPO_NAME SOURCE_COMMIT DK_TAG_FULL
+
+echo "Building branch $IMAGE_TAG as docker image $DK_TAG_FULL"
+docker build . -t "$DK_TAG_FULL" --build-arg SOURCE_COMMIT="${SOURCE_COMMIT}" --build-arg DOCKER_TAG="${IMAGE_TAG}"
+
+# Return to the CWD the user was in prior to running this script.
+cd - &> /dev/null
+