build: update dockerfiles to fix hadolint issues. #206

Job	Run time
test_course_definition / go_lint	4s
test_course_definition / js_lint	28s
test_course_definition / identify_language_slugs	3s
test_course_definition / docker_lint	10s
test_course_definition / markdown_lint	30s
test_course_definition / rust_lint	6s
test_course_definition / validate-schemas	22s
test_course_definition / validate-stages-naming	6s
test_course_definition / verify_templates_compiled	26s
test_course_definition / test (cpp)	43s
test_course_definition / test (go)	28s
test_course_definition / test (haskell)	1m 57s
test_course_definition / test (javascript)	25s
test_course_definition / test (python)	36s
test_course_definition / test (ruby)	26s
test_course_definition / test (rust)	51s
	7m 41s

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