build.yaml: drop tags from the per-platform push step (they conflict
with push-by-digest=true,push=true and cause all master-branch matrix
jobs to fail on GHCR). The merge job already creates the :build and
:<version>-build tags via buildx imagetools create. Also drop linux/arm/v7
from the matrix: proton-bridge's CGO deps (libfido2/cbor) don't
cross-compile cleanly on 32-bit ARM and upstream doesn't target it.

build/Dockerfile: add runtime libfido2-1 and libcbor0.10 (dynamically
linked by the built binaries, previously missing), add pkg-config to
the build stage (CGO needs it to find .pc files), add procps for the
healthcheck, and add a HEALTHCHECK so orchestrators detect a crashed
bridge. Pulled from shenxn#149, shenxn#134, shenxn#128.

build/entrypoint.sh, deb/entrypoint.sh: remove stale /root/.gnupg/
S.gpg-agent* sockets on startup (fixes bridge failing to start after
a container restart) and harden socat with fork,reuseaddr and nodelay
for more reliable port forwarding. Pulled from
shenxn#134.

README.md: drop arm/v7 from the supported platforms list.