COMP: Remove docker build warning

- LegacyKeyValueFormat: "ENV key=value" should be used
  instead of legacy "ENV key value" format (line 42)

"""
The new format for declaring environment variables in Dockerfiles is ENV
key=value. This format, where the key and value are separated by an
equals sign (=), is the recommended approach. The older format, ENV key
value (with a space as a separator), is deprecated.
""" -- google search

android-x86/Dockerfile

@@ -11,8 +11,8 @@ ENV AS=${CROSS_ROOT}/bin/llvm-as \
     CXX=${CROSS_ROOT}/bin/clang++ \
     LD=${CROSS_ROOT}/bin/ld
 
-ENV ANDROID_NDK_REVISION 25b
-ENV ANDROID_API 23
+ENV ANDROID_NDK_REVISION=25b
+ENV ANDROID_API=23
 
 RUN mkdir -p /build && \
     cd /build && \
@@ -36,7 +36,7 @@ COPY config.toml /root/.cargo/
 
 # Prepare CMake
 COPY Toolchain.cmake ${CROSS_ROOT}/
-ENV CMAKE_TOOLCHAIN_FILE ${CROSS_ROOT}/Toolchain.cmake
+ENV CMAKE_TOOLCHAIN_FILE=${CROSS_ROOT}/Toolchain.cmake
 
 # Build-time metadata as defined at http://label-schema.org
 ARG BUILD_DATE
@@ -50,4 +50,4 @@ LABEL org.label-schema.build-date=$BUILD_DATE \
       org.label-schema.vcs-ref=$VCS_REF \
       org.label-schema.vcs-url=$VCS_URL \
       org.label-schema.schema-version="1.0"
-ENV DEFAULT_DOCKCROSS_IMAGE ${IMAGE}:${VERSION}
+ENV DEFAULT_DOCKCROSS_IMAGE=${IMAGE}:${VERSION}