Modify parsing of tags

[JCGoran]

Fixes #41.
Instead of Git tags, it now parses Dockerhub tags instead.
Additionally, now it always uses 'latest' as the default tag.

[schuellerf]

The changes do make sense to me and I'll test and merge if I find more time than today...

[schuellerf]

I prefer if it can be overridden if needed

Suggested change

	TAG='latest'
	TAG=${TAG:-latest}

[JCGoran]

You're absolutely right, fixing it now.

[schuellerf]

You could move the error message and printing of all tags which are available here.
(or you wanted to make the call more encapsulated?)
Either way I think printing the list of available tags, if not found, would improve the usability alot

[JCGoran]

Okay, your first suggestion was simple to fix, but the second was a bit trickier, and I will admit that my solution is somewhat hacky.
That said, the following used to break the script:

# invalid tag (unless it's available locally)
$ export TAG='asdf' && ./xfce-test start
# invalid Dockerhub tag
$ ./xfce-test pull asdf

Now it doesn't break anymore, and the output with an invalid tag looks something like this:

$ export TAG='asdf' && ./xfce-test start
The tag 'asdf' was not found on Dockerhub
---
The following tags are available on Dockerhub:
latest
audio_test
...
ubuntu_20.04-xfce-4.16pre2
ubuntu_xfce-4.16pre1
xubuntu_19.04
Attempting to find the tag 'asdf' locally...
The tag 'asdf' was not found locally
---
The following tags are available locally:
latest
ubuntu_20.04-xfce-4.16pre2
Unable to find the tag 'asdf' either on Dockerhub or locally, exiting...

while the pull version outputs just the Dockerhub tags and exits gracefully.
This is a bit lengthy, but could be fixed separately by adding a verbosity flag or something to the script itself.