Test

Update sample dockerfiles to use latest base images #124

Job	Run time
test_course_definition / js_lint	28s
test_course_definition / identify_language_slugs	2s
test_course_definition / go_lint	3s
test_course_definition / docker_lint	7s
test_course_definition / markdown_lint	24s
test_course_definition / validate-schemas	18s
test_course_definition / rust_lint	5s
test_course_definition / validate-stages-naming	9s
test_course_definition / verify_templates_compiled	19s
test_course_definition / test (c)	26s
test_course_definition / test (go)	43s
test_course_definition / test (nim)	41s
test_course_definition / test (php)	31s
test_course_definition / test (python)	23s
test_course_definition / test (ruby)	32s
test_course_definition / test (rust)	1m 24s
test_course_definition / test (swift)	53s
	7m 28s

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