add Singularity support for "dockerFile" #1875

Job	Run time
cwltool release test	7m 21s
No leftovers	4m 37s
CWL conformance (v1.0, docker)	3m 58s
CWL conformance (v1.0, singularity)	3m 43s
Tox (3, 8, lint)	1s
Linters (lintreadme)	1m 20s
Tox (3, 8, unit)	1s
CWL conformance (v1.0, podman)	3m 47s
Linters (shellcheck)	18s
CWL conformance (v1.1, docker)	4m 1s
Tox (3, 8, bandit)	1s
Linters (pydocstyle)	14s
CWL conformance (v1.1, singularity)	3m 59s
Tox (3, 8, mypy)	1s
CWL conformance (v1.1, podman)	4m 3s
Tox (3, 9, lint)	1s
CWL conformance (v1.2, docker)	4m 0s
build_test_container	1m 31s
Tox (3, 9, unit)	1s
CWL conformance (v1.2, singularity)	3m 59s
Tox (3, 9, bandit)	1s
CWL conformance (v1.2, podman)	3m 58s
Tox (3, 9, mypy)	1s
Test on macos-latest	5m 27s
CWL conformance (v1.2, docker, --fast-parser)	3m 59s
Tox (3, 10, lint)	41s
Tox (3, 10, unit)	1s
Tox (3, 10, bandit)	38s
Tox (3, 10, mypy)	26s
Tox (3, 11, lint)	1s
Tox (3, 11, unit)	1s
Tox (3, 11, bandit)	28s
Tox (3, 11, mypy)	1s
Tox (3, 12, lint)	1s
Tox (3, 12, unit)	1s
Tox (3, 12, bandit)	1s
Tox (3, 12, mypy)	1s
	1h 2m 44s

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