Update Dockerfile

[joanise]

Our Dockerfile is out of date in several ways:

• image is old
• the instructions in README.md and Dockerfile don't quite align
• docker build . issues eight warnings.

warnings from build:

8 warnings found (use docker --debug to expand):
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 28)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 35)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 36)
 - JSONArgsRecommended: JSON arguments recommended for CMD to prevent unintended behavior related to OS signals (line 38)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 1)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 8)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 3)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 27)

Update the Dockerfile, or delete it if we don't plan to maintain it.