Optimized Docker build script for faster rebuilds

[maciej-trebacz]

This script creates a cache for Go modules, significantly speeding up the Docker build (from ~40 seconds to ~3s)

[decomp-dev]

Report for us (91868eb - 02cef32)

[!] Report not found. Did the build succeed?

[Xeeynamo]

./Dockerfile
# 2. launch container and mount current directory under /ff7
# docker run --name ff7-work -it -v $(pwd):/ff7 -v ff7_venv:/ff7/.venv -v ff7_build:/ff7/build ff7-build

Let's integrate these flags from the original Dockerfile instructions:

• --name ff7-work: useful for docker start -ai ff7-work
• -v ff7_build:/ff7/build: useful to produce the binaries on the host

In addition I suggest to:

• -v go_cache:/gocache: instead of mounting ~/.cache to avoid writing files on the host. For Docker on macOS, it's faster to have large cache volumes within the Docker VM than mapped from the host. This also allows to remove mkdir -p ~/.cache/ff7-go-mod ~/.cache/ff7-go-build.
• -e "GOMODCACHE=/gocache/mod" and similar: shall we move these two env variables in the Dockerfile? So tools/docker-build.sh gets leaner.
• Let's update the Dockerfile instructions in the comments to point to this newer shell script!

[maciej-trebacz]

-v ff7_build:/ff7/build: useful to produce the binaries on the host

In the original PR I've opted for mounting the build directory to the one in the project root on the host, it made it very easy to test built images, but this works too.

[Xeeynamo]

Let's not have the agent instructions merged in main for now (see feedback here: #17 (comment))

[Xeeynamo]

Looks good, thanks! And sorry for the late review