Configure Dependabot for Docker image

[pjonsson]

Reason for this pull request

This will cause Dependabot to make
pull requests for the Dockerfile
when there are new releases of GDAL.

Proposed changes

Repo-internal change to make Dependabot submit pull requests for the Dockerfile.

• Closes #xxxx
• Tests added / passed
• Fully documented, including docs/about/whats_new.rst for all changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.78%. Comparing base (81b4a31) to head (a3bce44).
Report is 5 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1624   +/-   ##
========================================
  Coverage    91.78%   91.78%           
========================================
  Files          132      132           
  Lines        14640    14640           
========================================
  Hits         13437    13437           
  Misses        1203     1203           

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