Upgrade alpine from 3.8 to 3.10 in armhf Dockerfile

Upgrade alpine from 3.8 to 3.10 in armhf Dockerfile

The Docker for armhf doesn't build anymore on Alpine 3.8, upgrading to 3.10.
Building works fine on a Rapsberry Pi 4 running Raspbian GNU/Linux 10 (buster)

This is the error with 3.8:
```
[2/4] Fetching packages...
error css-loader@4.3.0: The engine "node" is incompatible with this module. Expected version ">= 10.13.0".
error Found incompatible module
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.
The command '/bin/sh -c apk --update --no-cache add yarn nodejs-current python2 build-base     && cd /shaarli     && yarn install     && yarn run build     && rm -rf node_modules' returned a non-zero code: 1
```

Not upgrading to 3.11, due to this error:
```
2/4] Fetching packages...
error browserslist@4.14.3: The engine "node" is incompatible with this module. Expected version "^6 || ^7 || ^8 || ^9 || ^10 || ^11 || ^12 || >=13.7". Got "13.1.0"
error Found incompatible module.
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.

The command '/bin/sh -c apk --update --no-cache add yarn nodejs-current python2 build-base     && cd /shaarli     && yarn install     && yarn run build     && rm -rf node_modules' returned a non-zero code: 1
```

Not upgrading to 3.12 either, due to this error:
```
ERROR: unsatisfiable constraints:
  py2-pip (missing):
    required by: world[py2-pip]
The command '/bin/sh -c apk --update --no-cache add py2-pip     && cd /usr/src/app/shaarli     && pip install --no-cache-dir mkdocs     && mkdocs build --clean' returned a non-zero code: 1
```
There's probably a way to have Python2 pip installed on 3.12, but I suppose other issues would arise (such as the one happening with 3.11), so only proposing to upgrade to 3.10 now. This would probably be looked at more in detail when merging the amd64 and arm/v7 Docker builds, see #1496.