Migrate docker image off geobase to osgeo/gdal

[SpacemanPaul]

Reason for this pull request

Docker image used for testing was built on datacube/geobase which is now deprecated.

Proposed changes

New Docker build based on osgeo/gdal. Simple single-stage docker build, could be further optimised but seems sufficient for CI testing and dev purposes.

• Closes Migrate Github test environment off geobase #1232
• Tests added / passed
• Fully documented, including docs/about/whats_new.rst for all changes

[codecov]

Codecov Report

Merging #1233 (e5670c6) into develop (0fc51cd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1233   +/-   ##
========================================
  Coverage    93.79%   93.79%           
========================================
  Files          103      103           
  Lines        10597    10597           
========================================
  Hits          9939     9939           
  Misses         658      658           

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[alexgleith]

Only change needed really is to pin the FROM docker image version.

[alexgleith]

Looks good.