Use dockerfile from respective fork or branch for CI runs

[IshaanDesai]

Currently the dockerfile used in CI runs is precice/ci-spack-pyprecice-deps-1804 which is the dockerfile on python-bindings/develop whereas actually the dockerfile from the respective fork or branch needs to be used in CI runs, for example this file from a recent PR. This happens because the file tag is hard coded in the workflow files:

https://github.com/precice/python-bindings/blob/36e4782b8460ba93ec6bfc307bef7bf281c540fa/.github/workflows/build-env.yml#L28

https://github.com/precice/python-bindings/blob/36e4782b8460ba93ec6bfc307bef7bf281c540fa/.github/workflows/build-spack.yml#L17