Merge pull request from GHSA-3qm5-5hmp-8c6w

* Limit the size of the content type definition in OOXML

* Improve naming in OOXML tests

* Check OOXML error type in tests

docd: fix Docker workflow (#160)

add linux/arm64 platform to docd image (#159)

Refs #142

docd/appengine: bump to 1.3.9

docd: add slog and JSON logging (#149)

docd: bump to Go 1.21 and debian:12 (#145)

Also show Go version in startup logs.

all: remove refs to deprecated io/ioutil (#140)

docd: install ca-certificates (#130)

docd: fallback to AppEngine env vars when flags not provided (#128)

docd: return JSON errors when things fail (#127)