Add POSTGRES_INITDB_ARGS env var

[knickers]

This closes #19 per @tianon's suggestion.

$ docker run --name some-postgres -e POSTGRES_PASSWORD=mysecretpassword -e POSTGRES_INITDB_ARGS="--data-checksums" --rm postgres
The files belonging to this database system will be owned by user "postgres".
This user must also own the server process.

The database cluster will be initialized with locale "en_US.utf8".
The default database encoding has accordingly been set to "UTF8".
The default text search configuration will be set to "english".

Data page checksums are enabled.
...

[tianon]

I think this will cause trouble if we need multiple arguments (ie, something like POSTGRES_INITDB_ARGS='--a --b --c --d="e f").

Something like this is probably what we'll have to do instead: (so we get quote parsing, etc as appropriate)

eval "gosu postgres initdb $POSTGRES_INITDB_ARGS"

[knickers]

This also successfully enables data page checksums, but I have not tested it simultaneously with any other arguments.

[yosifkit]

LGTM

[tianon]

LGTM 👍