add Singularity support for "dockerFile" #1936

Job	Run time
build_test_container	1m 4s
No leftovers	5m 16s
CWL conformance (v1.0, docker)	3m 45s
Test on macos-latest	4m 52s
CWL conformance (v1.0, singularity)	3m 22s
cwltool release test	7m 40s
CWL conformance (v1.0, podman)	3m 32s
Tox (3, 8, lint)	1s
CWL conformance (v1.1, docker)	4m 55s
Tox (3, 8, unit)	1s
Linters (lintreadme)	1m 0s
CWL conformance (v1.1, singularity)	6m 16s
Tox (3, 8, bandit)	1s
Linters (shellcheck)	19s
CWL conformance (v1.1, podman)	5m 18s
Tox (3, 8, mypy)	1s
Linters (pydocstyle)	30s
CWL conformance (v1.2, docker)	6m 53s
Tox (3, 9, lint)	1s
CWL conformance (v1.2, singularity)	6m 55s
CWL conformance (v1.2, podman)	7m 1s
Tox (3, 9, unit)	1s
CWL conformance (v1.2, docker, --fast-parser)	7m 1s
Tox (3, 9, bandit)	1s
Tox (3, 9, mypy)	1s
Tox (3, 10, lint)	30s
Tox (3, 10, unit)	1m 21s
Tox (3, 10, bandit)	25s
Tox (3, 10, mypy)	1m 4s
Tox (3, 11, lint)	25s
Tox (3, 11, unit)	46s
Tox (3, 11, bandit)	31s
Tox (3, 11, mypy)	49s
Tox (3, 12, lint)	1s
Tox (3, 12, unit)	1s
Tox (3, 12, bandit)	1s
Tox (3, 12, mypy)	1s
	1h 21m 42s

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