Bump s6-overlay to v3.2.0.0 and fix env warnings

[xlionjuan]

• Bump s6-overlay to v3.2.0.0.
• Fix LegacyKeyValueFormat warnings

Error will shown while building:

 2 warnings found (use docker --debug to expand):
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 15)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 16)

Solution: Add = between env name and value

ENV RELAY=relay.example.com
ENV ENCRYPTED_ONLY=0