Docker build uses configurable FETCH_VERSION.

[spoulson]

Build specific versions instead of "latest" by providing an optional FETCH_VERSION build arg.

cd amd64

# Build latest.
docker build -t observium .

# Build 22.12.
docker build -t observium:22.12 --build-arg FETCH_VERSION=22.12 .

FETCH_VERSION may be any version tagged on the published tarball at:
http://www.observium.org/observium-community-${FETCH_VERSION}.tar.gz

Included some config tidy on incorrect indentation.

Closes #29

[abnerrizzi]

LGTM

[somsakc]

Let's me take a time to review your pull request and assess for other impact. Thank for your contributions..