Add initial jq-based templating engine

[yosifkit]

Similar to docker-library/php#1052.

This also takes some bits (like dirCommit and Dockerfile.debian) from #680 in order to support that PR easier.

The new Dockerfile template should make it easier to keep the version-specific Dockerfiles in sync.

--- ../official-images/library/mysql	2020-10-22 15:16:01.132359192 -0700
+++ /dev/fd/63	2020-11-18 15:46:54.728546973 -0800
@@ -1,17 +1,20 @@
-# this file is generated via https://github.com/docker-library/mysql/blob/ff627942605ce08a32fa527993337df0e9856760/generate-stackbrew-library.sh
+# this file is generated via https://github.com/docker-library/mysql/blob/e0ac64268f0f2d151baea664b5fa9dbcaa0c9321/generate-stackbrew-library.sh
 
 Maintainers: Tianon Gravi <[email protected]> (@tianon),
              Joseph Ferguson <[email protected]> (@yosifkit)
 GitRepo: https://github.com/docker-library/mysql.git
 
 Tags: 8.0.22, 8.0, 8, latest
-GitCommit: 1a25a09159a3d7b6b5b8ad0e6c7eb53504b3aab5
+GitCommit: e0ac64268f0f2d151baea664b5fa9dbcaa0c9321
 Directory: 8.0
+File: Dockerfile.debian
 
 Tags: 5.7.32, 5.7, 5
-GitCommit: 1a25a09159a3d7b6b5b8ad0e6c7eb53504b3aab5
+GitCommit: e0ac64268f0f2d151baea664b5fa9dbcaa0c9321
 Directory: 5.7
+File: Dockerfile.debian
 
 Tags: 5.6.50, 5.6
-GitCommit: 1a25a09159a3d7b6b5b8ad0e6c7eb53504b3aab5
+GitCommit: e0ac64268f0f2d151baea664b5fa9dbcaa0c9321
 Directory: 5.6
+File: Dockerfile.debian

[tianon]

Overall looks good, nice work!

Just a few (minor) comments. 👍

[tianon]

If this jq fails for some reason (syntax error, .[env.version] being undefined, etc), then this script will silently skip this $version instead of also failing, so I think we need to split this out:

Suggested change

	if [ -n "$(jq -r '.[env.version].debian // ""' versions.json)" ]; then
	debianVersion="$(jq -r '.[env.version].debian // ""' versions.json)"
	if [ -n "$debianVersion" ]; then

Alternatively, we could (should?) ditch this conditional completely and just let the template generate always -- if we someday remove/deprecate the "debian" variant in a future major/minor release post-#680, then we would introduce a check.

[yosifkit]

Yeah, the if was inspired by a similar section in 680: https://github.com/docker-library/mysql/pull/680/files#diff-e14594f6bb9b29ac0146c04e78ae33e8e92f2f00709fefab97359dea31b94d11R98-R127. It seems fine to keep.

[tianon]

(I look forward to rebasing #680 😆)

[tianon]

Looks great -- actual Dockerfile changes are very minimal, and this improves the maintainability (especially the ability to verify PRs are updating templates properly and that the CI is testing the PR changes and that @docker-library-bot won't unexpectedly revert or change things post-merge).