Test

Update sample dockerfiles to use latest base images #125

Job	Run time
test_course_definition / go_lint	3s
test_course_definition / js_lint	22s
test_course_definition / identify_language_slugs	3s
test_course_definition / docker_lint	6s
test_course_definition / markdown_lint	23s
test_course_definition / rust_lint	6s
test_course_definition / validate-schemas	18s
test_course_definition / validate-stages-naming	6s
test_course_definition / verify_templates_compiled	19s
test_course_definition / test (c)	28s
test_course_definition / test (go)	43s
test_course_definition / test (nim)	31s
test_course_definition / test (php)	31s
test_course_definition / test (python)	29s
test_course_definition / test (ruby)	26s
test_course_definition / test (rust)	1m 24s
test_course_definition / test (swift)	1m 2s
	7m 20s

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