Update dockerfiles to support emsdk

[mattsoulanille]

Update dockerfiles to support Emscripten's official Bazel toolchain, which #4769 adds.

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This change is 

[mattsoulanille]

I've tested building the containers locally, and I've built parts of tfjs with them, but I haven't done a full rebuild. I don't expect these changes to break anything since they only add new dependencies to the dockerfiles.

[lina128]

Reviewable status: 0 of 1 approvals obtained (waiting on @mattsoulanille)

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dockers/wasm-docker/Dockerfile, line 8 at r1 (raw file):

RUN apt-get update -qqy && apt-get install -qqy \
        curl \

Do you need all of this? Better to just pick the ones you need. Otherwise, it increases docker building time at the beginning of each CI run.

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dockers/wasm-docker/Dockerfile, line 21 at r1 (raw file):

pip3 install -U absl-py

Can you follow the same style as the release docker:

# Install absl
RUN pip3 install -U absl-py

[lina128]

LGTM

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @mattsoulanille)