Address build warnings

[lukebakken]

The following warnings are emitted by docker build:

 19 warnings found (use docker --debug to expand):
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 204)
 - SecretsUsedInArgOrEnv: Do not use ARG or ENV instructions for sensitive data (ENV "RABBITMQ_PGP_KEY_ID") (line 235)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 239)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 9)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 32)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 199)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 206)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 235)
 - SecretsUsedInArgOrEnv: Do not use ARG or ENV instructions for sensitive data (ENV "OPENSSL_PGP_KEY_IDS") (line 35)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 37)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 43)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 44)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 233)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 236)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 305)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 22)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 192)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 200)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 115)

This PR addresses them.

[tianon]

I don't put much stock in any of these (and have consistently and actively opposed the implementation of one in particular: moby/buildkit#5130), but I won't block this if you feel strongly about them.

Some further added context:

• the official downstream builds disable all of these: https://github.com/docker-library/meta-scripts/blob/af716438af4178d318a03f4144668d76c9c8222f/meta.jq#L190 (it's on our TODO list to apply this to the CI/PR smoke tests too, but it's one of many yaks)

• the BuildKit frontend we actually use for these builds downstream patches the feature out completely: https://github.com/tianon/dockerfiles/blob/ac08ffa5a3c2bdb0559765b2d864db3a88c7f86b/buildkit/nolint.patch

[lukebakken]

Thanks for the context links. I don't feel strongly about them but I also don't like seeing warnings when I build containers to test PRs here 😸