Adds Docker build and GHA test of container

[tomaroberts]

PR adds:

• Dockerfile to build nii2dcm
• GHA which tests the build on PR

TODO:

• [x] do not merge until updated Dockerfile with pip install PyPI version
• [x] add Docker section to README
• [x] push container to GHCR

[tomaroberts]

Tests fine, failed because of TestPyPI timeout.

[github-actions[bot]]

Coverage Report

File	Stmts	Miss	Cover	Missing
nii2dcm
__main__.py	34	34	0%	5–68
dcm.py	93	2	98%	165–166
dcm_writer.py	33	6	82%	89–95
nii.py	35	2	94%	34–35
run.py	30	2	93%	57, 66
utils.py	7	7	0%	8–36
nii2dcm/modules
mr_image.py	59	2	97%	39–40
TOTAL	431	55	87%

Tests	Skipped	Failures	Errors	Time
18	0 :zzz:	0 :x:	0 :fire:	4.993s :stopwatch:

[github-actions[bot]]

Coverage Report

File	Stmts	Miss	Cover	Missing
nii2dcm
__main__.py	34	34	0%	5–68
dcm.py	93	2	98%	165–166
dcm_writer.py	33	6	82%	89–95
nii.py	35	2	94%	34–35
run.py	30	2	93%	57, 66
utils.py	7	7	0%	8–36
nii2dcm/modules
mr_image.py	59	2	97%	39–40
TOTAL	431	55	87%

Tests	Skipped	Failures	Errors	Time
18	0 :zzz:	0 :x:	0 :fire:	4.925s :stopwatch: