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.