Attempt fix for staging deployment

[MattPereira]

Fixes #457

Changes

• Added postgresql-dev to the dockerfile that build image used for staging site deployment
• I think the root of problem is that pg_config cannot be found and GPT thinks that installing postgresql-dev as part of the image building process will make pg_config available

[bzzz-coding]

Let's give this a try!

[alexlaw528]

Seems like a sensible solution. Let's give it a shot!