Use NCEPLIBS-g2c v2.1.0 + enable wgrib2 PNG decoder

The linker error from the previous attempt was actual symbol
references, not a flag-combination oddity:

  undefined reference to `g2c_enc_jpeg2000'
  undefined reference to `g2c_dec_jpeg2000'

Those live in g2c's "new" file-based API, built only when
BUILD_G2C=ON. In v1.9.0 the option defaulted OFF and was marked
"experimental until 2.0.0 release", so I had explicitly disabled
it. In v2.1.0 (released 2025-01-13, before wgrib2 v3.6.0 on
2025-02-19) it defaults ON and the API is stable. Bump
G2C_VERSION to 2.1.0 and drop the BUILD_G2C override.

With the new-API symbols now exported, wgrib2 can be compiled
with every decoder we care about in one shot:

  -DUSE_PNG=ON -DUSE_OPENJPEG=ON -DUSE_AEC=ON

`wgrib2 -config` in the final runtime image now reports:

  Supported decoding: simple, complex, rle, ieee, png, jpeg2000,
                      CCSDS AEC

Verified end-to-end by building the full image with podman and
running wgrib2 -config inside it. MrmsFetchWorker's packing
type 41 (PNG) messages will decode once the rolling deploy lands.

Dockerfile

@@ -22,7 +22,12 @@ ARG RUNNER_IMAGE="docker.io/debian:${DEBIAN_VERSION}"
 FROM ${RUNNER_IMAGE} AS wgrib2-builder
 
 ARG WGRIB2_VERSION=3.6.0
-ARG G2C_VERSION=1.9.0
+# g2c v2.1.0 is the first release where BUILD_G2C defaults ON, which
+# is what wgrib2 v3.6.0 needs at link time (g2c_enc_jpeg2000 /
+# g2c_dec_jpeg2000 come from the "new" file-based API). v1.9.0's
+# same flag was OFF by default with a note about the API being
+# experimental until 2.0.0.
+ARG G2C_VERSION=2.1.0
 
 RUN apt-get update \
   && apt-get install -y --no-install-recommends \
@@ -54,24 +59,20 @@ RUN wget -q --content-disposition "https://github.com/NOAA-EMC/NCEPLIBS-g2c/arch
 
 WORKDIR /tmp/wgrib2
 
-# wgrib2 flag notes:
-#   * USE_OPENJPEG is what wgrib2's own CI (Linux_options.yml) tests
-#     — it triggers the g2c linkage and is known to build cleanly. An
-#     earlier attempt with -DUSE_PNG=ON alone got all the way to
-#     linking libwgrib2.so and then died with Error 2 on the wgrib2_exe
-#     link step (wgrib2 doesn't gate every jpeg_pk.c path properly
-#     when USE_PNG is set without USE_JASPER/USE_OPENJPEG).
-#   * wgrib2 delegates actual decompression to g2c, and g2c above was
-#     built with USE_PNG=ON, so PNG-packed GRIB2 (DRT 5.41, e.g. MRMS
-#     PrecipRate) still decodes correctly through this link even
-#     though wgrib2's own USE_PNG knob stays off.
+# wgrib2 flags:
+#   * USE_PNG / USE_OPENJPEG / USE_AEC each turn on the matching
+#     encoder path inside wgrib2 and add the compression name to the
+#     -config output. With g2c v2.1.0 underneath (which exports the
+#     g2c_enc_* / g2c_dec_* new-API symbols) all three can be enabled
+#     together — the earlier build failure was not about the flag
+#     combination, it was g2c v1.9.0 shipping without BUILD_G2C.
 RUN wget -q --content-disposition "https://github.com/NOAA-EMC/wgrib2/archive/refs/tags/v${WGRIB2_VERSION}.tar.gz" \
       -O wgrib2.tar.gz \
   && tar xzf wgrib2.tar.gz \
   && cd wgrib2-${WGRIB2_VERSION} \
   && mkdir build && cd build \
   && cmake .. -DCMAKE_INSTALL_PREFIX=/usr/local -DCMAKE_BUILD_TYPE=Release \
-       -DUSE_OPENJPEG=ON -DUSE_AEC=ON \
+       -DUSE_PNG=ON -DUSE_OPENJPEG=ON -DUSE_AEC=ON \
   && make -j$(nproc) \
   && make install \
   && strip /usr/local/bin/wgrib2