docker: pin version of alpine for ctags build
jansson-dev in Alpine 3.12 no longer includes a static archive [1]. This
broke the build for ctags. To workaround this we can use an older
version of alpine which does include the static archive.
[1]: https://gitlab.alpinelinux.org/alpine/aports/-/commit/569cdb02b4418a574184113b313b5fd89893e8f7
Change-Id: I83355f06c9b3a64036e9a2fd69354b49549f87d6