Add git to build dependencies in Dockerfile

[dweinholz]

Added git to the list of build dependencies in Dockerfile.

Try to fulfill the following points before the Pull Request is merged:

Codebase and Testing

• Code changes are covered with unit tests
• For changes in DB: migrations are made and pushed to the branch
• The dev branch is merged into the current branch
• If a linting PR exists, it must be merged before this PR is allowed to be merged.

Documentation

• Check if anything in the Readme must be adjusted (development-, production-, setup).
• Check if any section in the wiki (https://simplevm.denbi.de/wiki/) should be adjusted.
• If code is unreadable and not well commented: Open commenting issue with tag "important"

Security

• For any new features or security changes, check against the OWASP guidelines for possible vulnerabilities
• For any updates in packages and requirements: Check if the patches still work

For releases only:

• If the review of this PR is approved and the PR is followed by a release then the .env file
  in the cloud-portal repo should also be updated.
• If you are making a release then please sum up the changes since the last release on the release page using the clog tool with clog -F

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
simple_vm_client
VirtualMachineHandler.py	226	22	22	90%
__init__.py	1	0	100%
simple_vm_client/benchmark
__init__.py	0	0	100%
test_service.py	52	36	36	31%
simple_vm_client/bibigrid_connector
__init__.py	0	0	100%
bibigrid_connector.py	135	36	36	73%
test_bibigrid_connector.py	119	0	100%
simple_vm_client/flavor_resource_exporter_connector
__init__.py	2	0	100%
flavor_resource_exporter_connector.py	131	17	17	87%
test_flavor_resource_exporter_connector.py	242	0	100%
simple_vm_client/forc_connector
__init__.py	0	0	100%
forc_connector.py	336	32	32	90%
simple_vm_client/forc_connector/playbook
__init__.py	0	0	100%
playbook.py	216	3	3	99%
test_playbook.py	424	0	100%
simple_vm_client/forc_connector/template
__init__.py	0	0	100%
template.py	229	14	14	94%
simple_vm_client/metadata_connector
metadata_connector.py	112	26	26	77%
test_metadata_connector.py	68	0	100%
simple_vm_client/openstack_connector
__init__.py	0	0	100%
openstack_connector.py	1279	236	236	82%
test_openstack_connector.py	1376	2	2	99%
simple_vm_client/tests
test_virtualmachinehandler.py	325	0	100%
simple_vm_client/util
__init__.py	0	0	100%
state_enums.py	70	0	100%
test_thrift_converter.py	170	0	100%
thrift_converter.py	80	3	3	96%
TOTAL	5593	427	92%

Tests	Skipped	Failures	Errors	Time
350	0 💤	0 ❌	0 🔥	6.066s ⏱️