add Singularity support for "dockerFile" #1900

Job	Run time
build_test_container	9m 8s
No leftovers	3m 55s
CWL conformance (v1.0, docker)	3m 45s
Test on macos-latest	6m 44s
CWL conformance (v1.0, singularity)	3m 18s
cwltool release test	6m 48s
CWL conformance (v1.0, podman)	3m 39s
Tox (3, 8, lint)	29s
CWL conformance (v1.1, docker)	4m 43s
Linters (lintreadme)	1m 11s
Tox (3, 8, unit)	2m 28s
CWL conformance (v1.1, singularity)	4m 29s
Linters (shellcheck)	11s
Tox (3, 8, bandit)	31s
CWL conformance (v1.1, podman)	4m 46s
Linters (pydocstyle)	16s
CWL conformance (v1.2, docker)	4m 47s
Tox (3, 8, mypy)	2m 55s
CWL conformance (v1.2, singularity)	4m 44s
Tox (3, 9, lint)	30s
Tox (3, 9, unit)	1m 54s
CWL conformance (v1.2, podman)	4m 50s
CWL conformance (v1.2, docker, --fast-parser)	4m 45s
Tox (3, 9, bandit)	26s
Tox (3, 9, mypy)	2m 13s
Tox (3, 10, lint)	28s
Tox (3, 10, unit)	6m 29s
Tox (3, 10, bandit)	30s
Tox (3, 10, mypy)	5m 26s
Tox (3, 11, lint)	31s
Tox (3, 11, unit)	5m 59s
Tox (3, 11, bandit)	26s
Tox (3, 11, mypy)	6m 48s
Tox (3, 12, lint)	33s
Tox (3, 12, unit)	4m 18s
Tox (3, 12, bandit)	1m 44s
Tox (3, 12, mypy)	6m 7s
	2h 2m 44s

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