Slightly more secure and much smaller docker image

[c2h5oh]

Uses Google distroless Debian image as base of the final image ( see https://github.com/googlecontainertools/distroless ).

The new image:

• is more secure, because it's been stripped off anything that it could have been leaving much smaller attack surface
• is rootless
• doesn't constantly trigger security scanners - a CVE in anything included in the image does that and this one contains next to nothing
• is almost 85% smaller (10.5 vs 65 MB)

Motivation: security scanner flagging 3 CVEs in unused parts of the image + security team giving us 3 days to either fix it or retire pgweb..