fix: Fixes #2464 Replace git use for finding project root. Dedicated dockerignore with .git/ for 277MB image

[ltfschoen]

• You added a brief description of the PR, e.g.:

• followed these instructions https://stackoverflow.com/a/57774684/3208553 to create a custom ./docker/substrate_builder.Dockerfile.dockerignore that is dedicated to ./docker/substrate_builder.Dockerfile with fallback to ./.dockerignore

• checked current disk usage:

$ du -hc --max-depth=1 .[^.]* * | grep [MG]
217M	.git/objects
218M	.git
16K	        .github/ISSUE_TEMPLATE
15G	        bin/node-template
4.6M	bin/node
5.4M	bin
7.6M	client/executor
1.2M	client/network
14M	        client
1.2M	frame/contracts
3.3M	frame/support
13M	        frame
4.0K	        HEADER-GPL3
36K	        LICENSE-GPL3
3.9M	primitives
4.0K	        README.md
2.8M	zombienet/0001-basic-warp-sync
2.8M	zombienet

• added contents of .dockerignore to ./docker/substrate_builder.Dockerfile.dockerignore,
  then added extra folders like .git/ to it too

• used a different approach to obtaining the PROJECT_ROOT that didn't involve using git command based on this stackoverflow suggestion https://stackoverflow.com/a/57774684/3208553

• built the image

cd docker
./build.sh

• inspect the size of the image

$ docker images

parity/substrate   latest    31c200b06ab8   15 minutes ago   277MB
parity/substrate   v3.0.0    31c200b06ab8   15 minutes ago   277MB

$ docker run --rm -it parity/substrate substrate --version
substrate 3.0.0-dev-unknown

$ docker run --rm -it parity/substrate subkey --version
subkey 2.0.2

$ docker run --rm -it parity/substrate node-template --version
node-template 4.0.0-dev-unknown

$ docker run --rm -it parity/substrate chain-spec-builder --help
<it worked and output help stuff>

• You mentioned a related issue if this PR related to it, e.g. Fixes #228 or Related #1337.
• You asked any particular reviewers to review. If you aren't sure, start with GH suggestions.
  TODO
• Your PR adheres the style guide
  • In particular, mind the maximal line length.
  • There is no commented code checked in unless necessary.
  • Any panickers have a proof or removed.

[ggwpez]

Uhm why is this MR in your repo?

[ltfschoen]

Uhm why is this MR in your repo?

sorry i should have tagged reviewers later... this is still a WIP with a draft PR description...

because it's based on branch 'luke/13071-fix-ldd-error', which is only a branch in ltfschoen/substrate, and is still an active PR into 'master' of paritytech/substrate, so if i try to create a dedicated PR into paritytech/substrate 'master' from ltfschoen/substrate branch 'luke/2464-optimise-docker' then it shows changes from all commits from both branch 'luke/13071-fix-ldd-error' and 'luke/2464-optimise-docker' combined (instead of only showing 'luke/2464-optimise-docker' changes), but if 'luke/13071-fix-ldd-error' gets merged into 'master' of paritytech/substrate, then i can create a PR directly into 'master' of paritytech/substrate and it'll only show the 'luke/2464-optimise-docker' changes.

so my plan was to wait until 'luke/13071-fix-ldd-error' was merged into 'master' of paritytech/substrate, and then create a PR from branch 'luke/2464-optimise-docker' of ltfschoen/substrate into 'master' of paritytech/substrate, but in the interim while its still in my repo i wrote a draft PR description.... in hindsight i probably should have added tags for reviewers later on when i'd actually created the PR into 'master' of paritytech/substrate

[ltfschoen]

Uhm why is this MR in your repo?

FYI, i tried to get it to work with Alpine Linux instead of Ubuntu, but i could only reduce it to 236MB instead of 277MB, and i could only get it to work if i disabled the ldd ... command because there are some issues with the Alpine libraries.
so we can't consider using Alpine until that's resolved. i've included steps i took here in the PR with the changes i made so far #2